Sorry I forgot,
I am using Filemaker pro 10 advanced, and windows XP.
You have several options for Hiding text in a field. (Unlike access you have to link your text box to a field.)
A conditional format is one option. You can set a conditional format that changes the text and/or fill color to match that of the layout background to hide or reveal the contents of the field.
You can also define a calculation field like this:
If (//condition that determines message should be visible ; "Your Message" ; "")
I think I'll be playing with the calculated field.