O co chodzi z <Window.DataContext>

<Window x:Class="Hierarchy.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:Hierarchy"
        mc:Ignorable="d"
        Title="MainWindow" Height="453.583" Width="525">
    <Window.DataContext>
        <local:ViewModel/>
    </Window.DataContext>
...
</Window>

namespace Hierarchy
{
    class ViewModel : INotifyPropertyChanged
    {
        public ViewModel()
        {
            TEST = "jubba";

            m_folders = new List<IFolder>();

            //add Root items
            Folders.Add(new Folder { FolderLabel = "Dummy1", FullPath = @"C:\dummy1" });
            Folders.Add(new Folder { FolderLabel = "Dummy2", FullPath = @"C:\dummy2" });
            Folders.Add(new Folder { FolderLabel = "Dummy3", FullPath = @"C:\dummy3" });
            Folders.Add(new Folder { FolderLabel = "Dummy4", FullPath = @"C:\dummy4" });

            //add sub items
            Folders[0].Folders.Add(new Folder { FolderLabel = "Dummy11", FullPath = @"C:\dummy11" });
            Folders[0].Folders.Add(new Folder { FolderLabel = "Dummy12", FullPath = @"C:\dummy12" });
            Folders[0].Folders.Add(new Folder { FolderLabel = "Dummy13", FullPath = @"C:\dummy13" });
            Folders[0].Folders.Add(new Folder { FolderLabel = "Dummy14", FullPath = @"C:\dummy14" });


            Folders[0].Folders.Add(new Folder { FolderLabel = "Dummy24", FullPath = @"C:\dummy14" });


            Folders[0].Folders[0].Folders.Add(new Folder { FolderLabel = "xxxxx", FullPath = @"C:\dummy14" });
            Folders[0].Folders[0].Folders.Add(new Folder { FolderLabel = "vcxvx", FullPath = @"C:\dummy14" });
            Folders[0].Folders[0].Folders.Add(new Folder { FolderLabel = "vvxcvxcvcx", FullPath = @"C:\dummy14" });

        }
        

        public string TEST { get; set; }


        private List<IFolder> m_folders;
        public List<IFolder> Folders
        {
            get { return m_folders; }
            set
            {
                m_folders = value;
                NotifiyPropertyChanged("Folders");
            }
        }

        void NotifiyPropertyChanged(string property)
        {
            if (PropertyChanged != null)
                PropertyChanged(this, new PropertyChangedEventArgs(property));
        }

        public event PropertyChangedEventHandler PropertyChanged;
    }
}

public class ActionCommand : ICommand
    {
        public event EventHandler CanExecuteChanged
        {
            add { CommandManager.RequerySuggested += value; }
            remove { CommandManager.RequerySuggested -= value; }
        }

        private Action methodToExecute;
        private Func<bool> canExecuteEvaluator;

        public ActionCommand(Action methodToExecute, Func<bool> canExecuteEvaluator)
        {
            this.methodToExecute = methodToExecute;
            this.canExecuteEvaluator = canExecuteEvaluator;
        }

        public ActionCommand(Action methodToExecute)
            : this(methodToExecute, null)
        {
        }

        public bool CanExecute(object parameter)
        {
            if (canExecuteEvaluator == null)
            {
                return true;
            }
            else
            {
                bool result = canExecuteEvaluator.Invoke();
                return result;
            }
        }

        public void Execute(object parameter)
        {
            methodToExecute.Invoke();
        }
    }

public ICommand AddFolderCommand { get; set; }

// gdzies np w konstruktorze
AddFolderCommand = new ActionCommand(AddFolder);
//

private void AddFolder()
{
... //tutaj Twój kod po naciśnięciu buttona
}

public class ActionCommand : ICommand
    {
        public event EventHandler CanExecuteChanged
        {
            add { CommandManager.RequerySuggested += value; }
            remove { CommandManager.RequerySuggested -= value; }
        }

        private Action methodToExecute;
        private Func<bool> canExecuteEvaluator;

        public ActionCommand(Action methodToExecute, Func<bool> canExecuteEvaluator)
        {
            this.methodToExecute = methodToExecute;
            this.canExecuteEvaluator = canExecuteEvaluator;
        }

        public ActionCommand(Action methodToExecute)
            : this(methodToExecute, null)
        {
        }

        public bool CanExecute(object parameter)
        {
            if (canExecuteEvaluator == null)
            {
                return true;
            }
            else
            {
                bool result = canExecuteEvaluator.Invoke();
                return result;
            }
        }

        public void Execute(object parameter)
        {
            methodToExecute.Invoke();
        }
    }

<Window x:Class="Hierarchy.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:Hierarchy"
        mc:Ignorable="d"
        Title="MainWindow" Height="453.583" Width="525">
    <Window.DataContext>
        <local:ViewModel/>
    </Window.DataContext>
    <Grid>
        <StackPanel>
            <!-- simple root level binding-->
            <TextBlock Text="Simple root binding}"/>
            <TreeView ItemsSource="{Binding Folders}" >
                <TreeView.ItemTemplate>
                    <DataTemplate>
                        <TreeViewItem Header="{Binding FolderLabel}"/>
                    </DataTemplate>
                </TreeView.ItemTemplate>
            </TreeView>

            <!-- hieracical binding -->
            <TextBlock Text="Hierarchical root binding}"/>
            <TreeView ItemsSource="{Binding Folders}" Height="200">

               
                <TreeView.ItemTemplate>

                    

                    <HierarchicalDataTemplate ItemsSource="{Binding Folders}" DataType="{x:Type local:IFolder}">
                        <TreeViewItem Header="{Binding FolderLabel}"/>
                    </HierarchicalDataTemplate>

                    

                </TreeView.ItemTemplate>
            </TreeView>
            <Button x:Name="button" Content="Button" Command="{Binding AddFolderCommand}"/>
            <Button x:Name="button1" Content="Button" />

        </StackPanel>
    </Grid>
</Window>

<Window x:Class="WpfApplicationTreeView.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApplicationTreeView"
        mc:Ignorable="d"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <TreeView x:Name="treeView" HorizontalAlignment="Left" Height="257" Margin="24,26,0,0" VerticalAlignment="Top" Width="297">
            <TreeViewItem Header="dsadsa"/>
            <TreeViewItem Header="dasdsadsa"/>
            <TreeViewItem Header="bvcbvcbcv"/>
        </TreeView>

    </Grid>
</Window>

...
<HierarchicalDataTemplate ItemsSource="{Binding Folders}">
                        <TreeViewItem>
                            <TreeViewItem.ItemTemplate>
                                <DataTemplate>
                                    <Label Content="{Binding FolderLabel}"
                                           Margin="0 0 0 0" />
                                </DataTemplate>
                            </TreeViewItem.ItemTemplate>
                        </TreeViewItem>
                    </HierarchicalDataTemplate>
...

> pozycją tekstu sterujesz Margin'em Label'a. Zera znaczą analogicznie LEFT TOP RIGHT BOTTOM. Przejdź jakiś kurs XAML'a


Ok wygląda coś tam lepiej. Czy ten template oznacza, że teraz dziaciami są elementy typu Label a przedtem były to TreeViewItem-y?