Use the button tool to add a button object ot your layout or select an object you want to turn into a button that's already on the layout and select button setup...
Select the Perform script option and click the specify... button to bring up the specify script dialog. In the bottom of this dialog is found a text box titled "optional script parameter". Enter "Next" in this box for a next button or enter "Prev" for a previous button.
Thanks for your answer! I had the whole script parameter concept backwards. I thought the calculation went in the "optional script parameter" box. I checked my books before posting but the examples seemed to gloss over what actually servered as the script parameter. I have the Filemaker 9 Developers Reference and it's helpful but usually leaves me wanting more. It would be great if there were a larger book - or a website - where more numerous and more extensive examples of ALL calculation and script steps could be posted. The old Scriptology book seemed to have a lot more examples...but things weren't as complex back in those days.
And here's another mystery: Special Edtion Using Filemaker 9 by Jesse Feiler is largely a word for word copy of Special Edition Using Filemaker 8 but there is no mention of the original authors: Love, Lane and Bowers. It's no big deal but it's curious to me. Must be one of those publishing industry things.