For simple help with data entry, I think the "Invoices" sample file that shipped with FileMaker 4 used an effective technique. Clicking the help button switched you to a help layout which duplicated the data entry form from which you came. The background was lightened to distinguish it from the data entry screen, and the fields were set to disallow entry. On top of the form, simple yellow text boxes (like Post-its) and lines acted as a call-outs or balloons, pointing to and explaining the controls.
The user could simply toggle between the view in which they entered data and the view which offered explanations about the controls. The help view even showed the data they had entered so far. No muss, no fuss.