Since you are adding the record using a different form, you current form
doesn't know about it yet.  You should requery your form in the Not In List
event after the record has been added in the add record form.  Then rather
than a DLookup, use the FindFirst method on the form's recordsetclone to
navigate to the newly added record.  Here is a modification of your current
code:

Private Sub Combo14_NotInList(NewData As String, Response As Integer)
Dim Msg As String

   If NewData = "" Then
       Exit Sub
   End If

   If MsgBox(NewData & "' is not in the list." & vbNewLine & vbNewLine & _
           "Do you want to add it?", vbQuestion + vbYesNo) = vbYes Then
       DoCmd.OpenForm "newpart", , , , acAdd, acDialog, NewData
       Me.Combo14.Undo
       DoCmd.OpenForm "newpart", , , , acAdd, acDialog, NewData
       Response = acDataErrAdded
       Me.Requery
       With Me.RecordsetClone
           .FindFirst "[ProductID] = """ & NewData & """"
           If Not .NoMatch Then
               Me.Bookmark = .Bookmark
           End If
       End With
   Else
       Me.Combo14.Undo
       Response = acDataErrContinue
    End If

End Sub

Notice I used the constants vbQuestion, vbYesNo, etc rather than their
values.  It makes the code much easier to read.  Also, no need to Dim a
variable for a line feed. there are at least two different intrinsic
constants that do that.  I prefer vbNewLine, because its purpose is obvious
to the reader.