This example assumes you are using one of the non-standard settings for ViewStyle (Grid, FreeForm, OutlookExpress).

You get get a reference to an UltraTreeNodeColumn by handling the ColumnSetGenerated event, and accessing the Columns collection of the UltraTreeColumnSet that gets passed to the event.

Example:
private void AddDropDown( UltraTreeNodeColumn column )
{
    //  Create and populate a ValueList.
    ValueList vl = new ValueList();
    vl.ValueListItems.Add( 1, "One" );
    vl.ValueListItems.Add( 2, "Two" );
    vl.ValueListItems.Add( 3, "Three" );
    vl.ValueListItems.Add( 4, "Four" );
    vl.ValueListItems.Add( 5, "Five" );

    //  Assign the ValueList to the ValueList property.
    column.ValueList = vl;

    //  Assign an EditorWithCombo to the Editor property.
    column.Editor = new EditorWithCombo();
}