try this link.
http://www.cyclingforums.com/t300650...ad-issues.html
I just got a Powertap SL. I was p o'd because it came with a serial port hook up instead of USB. (contrary to the review in velonews)I had to get a keyspan serial port adapter. It's my undrsatanding that you can get a powertap to work with a mac if you get a serial port adapter that's compatible OS X.