excel2003

I got this from ozgrid.
Private Sub Worksheet_Change(ByVal Target As Range)

Dim lReply As Long

If Target.Cells.Count 1 Then Exit Sub
If Target.Address = "$D$1" Then
If IsEmpty(Target) Then Exit Sub
If WorksheetFunction.CountIf(Range("MyNames"), Target) = 0
Then
lReply = MsgBox("Add " & Target & " to list", vbYesNo +
vbQuestion)
If lReply = vbYes Then

Range("MyNames").Cells(Range("MyNames").Rows.Count + 1, 1) = Target
End If
End If
End If
End Sub

How do I use this on a User Form

I created the list on "Lists" sheet in H9:H10
List name is ModSN
I substituted my range and list name in the above code and refers to
formula, but it doesn't work. I used my textbox's name in place of the
address.

Here's what I have placed in the module where the ComboBox code is.
(Doubleclicked the combo box and pasted the code and modified)

Private Sub frmProcessDataEntry_Change(ByVal Target As Range)

Dim lReply As Long

If Target.Cells.Count 1 Then Exit Sub
If Target.Address = "cbModSN" Then
If IsEmpty(Target) Then Exit Sub
If WorksheetFunction.CountIf(Range("ModSN"), Target) = 0 Then
lReply = MsgBox("Add " & Target & " to list", vbYesNo +
vbQuestion)
If lReply = vbYes Then
Range("ModSN").Cells(Range("ModSN").Rows.Count +
1, 1) = Target
End If
End If
End If
End Sub