Public Sub AddItem(ByVal itemId As String)
Dim root As XPathNavigator = MainDataSource.CreateNavigator()
Dim group1 As XPathNavigator = root.SelectSingleNode("//my:myFields/my:group1/my:group2/my:group3", NamespaceManager)
Dim field1 As XPathNavigator = root.SelectSingleNode("//my:myFields/my:group1/my:group2/my:group3/my:Model_ListBox", NamespaceManager) 

‘field1 is my multiple selection listbox, when I check the properties, I get “nothing”  I thought it was the 
‘usage SelectSingleNode?? When it gets to next line, this is when I get the error message see above.
'I thought I had to delcare field1 as XPathNodeIterator, but then newnode can't take on .Clone().

Dim newNode As XPathNavigator = field1.Clone()
newNode.SetValue(itemId)group1.AppendChild(newNode)
End Sub