I am creating a very basic, editable form in Acrobat for a client on an existing PDF that they sent to me. For one section of the form, I have created a check box... and what I want it to do... is when the checkbox is "checked," a value of $50 is automatically populated in a separate field. This way, when they indicate by checking the box, that they are going to attend this event, the amount is automatically entered into the next field.
Lori Defurio has published her Hands on Session from the Adobe MAX conference and she has an example doing just what you describe. You can get the PDF of the presentation from this page-
[url=http://www.acrobatusers.com/tech_corners/loris_corner/dear_lori/2007/hands_on_max_session/]http://www.acrobatusers.com/tech_corners/loris_corner/dear_lori/2007/hands_on_max_session/[/url]
Look for the Calculating Fields exercise, and the actual exercise files can be found as an attachment in the PDF.
Hope that helps,
Dimitri
WindJack Solutions
[url=http://www.windjack.com]www.windjack.com[/url]
Dimitri Munkirs
WindJack Solutions
pdfscripting.com