[Erel]

To limit it to two levels you can use something like:

Sub btnAdd_Click
    If IsNull(tree.SelectedNode) Then
        Msgbox("Please select a node.")
        Return
    End If
    node.Value = tree.SelectedNode
    If Not(node.IsRoot) Then
        Msgbox("too deep!")
    Else
        node.AddNewNode(txtName.Text)
    End If
    tree.ExpandAll
End Sub