A popup message will most easily be triggered by an event on a form.  The
opening screen of your app can do it, or use the "on current" event of any
form that is used to view the clients.  You could write some code to do a
dlookup on a table or query that has as one of it's output columns the
number of days left for any due date.  That can be done by simple
subtraction such as:   Now() - [whateverdate]

Easiest of all would be a simple report that opens in preview mode whenever
you start the app or whenever you ask for it.

personally I think popups are overrated. It's kind of "bells and whistles"
stuff.  Just run the report regularly.....