The Time&Dialog function palette has One Button Dialogs and Two Button Dialogs. For either dialog, you specify the message. In the One Button Dialog, the user clicks OK to continue. In the Two Button Dialog, you can specify the names of the buttons (Yes/No, Ok/Cancel, etc.) and see which button the user clicked.
To get to the dialogs, right-click on an open spot on your diagram, then goto the Time&Dialog palette, then look for One Button or Two Button dialogs.