Tarzan,
The trickiest part of this question is deciding when you want this check
to take place. I guess the Before Update event of the form may be a
likely contender.
If you are using a macro, just make a macro with a MsgBox action, and
enter the required message text. Then, enter the equivalent of this in
the Condition column of the macro design...
[NameOfTheTextbox] Is Null
(if you can't see the Condition column in the macro design window,
select Conditions from the View menu).
Then, close and save the macro, and then assign this macro to the
Property in form design of the event you want it to run on.
