I ran across another forum that had an example. Seems I just needed to have the page load the control before adding it to the content pane.
Dim newTab As Tab = tabPartProducts.Tabs.Add(New Tab)
With newTab
.Text = productsList.Value(item.ProductId)
Dim uc As UserControl = Page.LoadControl("~/controls/edit/MyControl.ascx")
.ContentPane.Children.Add(uc)
End With
Dale