Hi Andrew, thank you for your help on this matter. Your solution works.

However, we are trying to use MVVM, I am wondering how to set the ActiveNode in the viewmodel?

I changed your codes as the following:

XAML:

<Window x:Class="XamDataTreeActiveNodeCase.MainWindow"
               xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
               xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
               xmlns:ig="http://schemas.infragistics.com/xaml"
               Title="MainWindow" Height="350" Width="525">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition/>
            <RowDefinition Height="30"/>
        </Grid.RowDefinitions>
        <Button x:Name="SetActiveNodeButton" Command="{Binding NewCommand}" Content="Set Active Node" Grid.Row="1"/>

<ig:XamDataTree x:Name="XamDataTree1" ItemsSource="{Binding Data}" DisplayMemberPath="Name" ActiveDataItem="{Binding SelectedItem, Mode=TwoWay}"
                       ActiveNodeChanged="XamDataTree1_ActiveNodeChanged">

         <ig:XamDataTree.GlobalNodeLayouts>
                 <ig:NodeLayout Key="ParentLayout" TargetTypeName="SampleData" DisplayMemberPath="Name"/>
         </ig:XamDataTree.GlobalNodeLayouts>
        <ig:XamDataTree.SelectionSettings>
              <ig:TreeSelectionSettings NodeSelection="Single"/>
        </ig:XamDataTree.SelectionSettings>

</ig:XamDataTree>

    </Grid>
</Window>

Code beghind:

public MainWindow()
{
InitializeComponent();
this.DataContext = new ViewModel();

private void XamDataTree1_ActiveNodeChanged(object sender, Infragistics.Controls.Menus.ActiveNodeChangedEventArgs e)
{
XamDataTreeNode XDTN = e.NewActiveTreeNode;
XDTN.IsSelected = true;
}

}

ViewModel:

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Runtime.CompilerServices;
using Microsoft.Practices.Prism.Commands;

namespace XamDataTreeActiveNodeCase
{
public class ViewModel : INotifyPropertyChanged
{
string[] names = { "Andrew", "Alex", "Trish", "Valerie", "Jen", "Robert", "Steve", "Eric" };

public ObservableCollection<SampleData> Data { get; set; }

public ViewModel()
{
Data = new ObservableCollection<SampleData>();
for (int i = 0; i < names.Length; i++)
{
Data.Add(new SampleData()
{
Name = names[i],
ID = i
});
}

NewCommand = new DelegateCommand(AddNewDTO);
}

private void AddNewDTO()
{
SelectedItem = new SampleData() { ID = 0, Name = "Andrew" };
}

private SampleData selectedItem;
public SampleData SelectedItem
{
get { return selectedItem; }
set
{ selectedItem = value; RaisePropertyChanged(); }
}

public DelegateCommand NewCommand { get; set; }

#region INotifyPropertyChanged Implementation
public event PropertyChangedEventHandler PropertyChanged;
protected void RaisePropertyChanged([CallerMemberName] string caller = "")
{
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(caller));
}
}
#endregion
}
public class SampleData
{
public string Name { get; set; }
public int ID { get; set; }
}
}

When I set the SelectedItem in the viewmodel, the ActiveNodeChanged event is not firing. 

Thank you.