Here is Parallax's
"Stamp on Mac" pdf.
It says specifically to use the Keyspan adapter.
If you do a Google search for "USB DB9", you'll find many places to get various adapters. They even have a USB to PS/2 adapter. I wish I would have seen that sooner, I would have picked up a legacy-free Abit AT7 mainboard instead of the EPoX 8K3A+, seeing as I can't live without my old IBM 'buckling spring" PS/2 keyboard. The EPoX is a great board, regardless, though.
Definitely let us know how it all turns out. I hope to get a Mac laptop in the future and will need to be able to program a BASIC Stamp and other embedded controllers with it.