Patrick:
In the NotInList property for the box, add:
Dim db As Database, rs As Recordset, strMsg As String, NewItem As Variant
strMsg = "'" & NewData & "' is not in the list"
strMsg = strMsg & "@Do you want to add the item to the list?"
strMsg = strMsg & "@Click Yes to ADD or No to re-type it."
NewItem = NewData
If MsgBox(strMsg, vbQuestion + vbYesNo, "Add new item?"

= vbNo Then
Response = acDataErrContinue
Else
Set db = CurrentDb
Set rs = db.OpenRecordset("YOUR TABLE NAME", dbOpenDynaset)
On Error Resume Next
rs.AddNew
rs!FIELD NAME = NewItem
rs.Update
If Err Then
MsgBox "An error occurred. Please try again."
Response = acDataErrContinue
Else
Response = acDataErrAdded
End If
End If
This should work. "Get it right the first time, that's the main thing..."