rusty,
you need to have VMWare (or similar product) installed then you install a *purchased* copy of windows onto it. Ie, you still have to buy windows (or at least have access to an install disc).
Then you start it up have what looks like a windows box running on your mac.
It's very disgusting... :-)
Also, there is a cultist plugin for Sketchup on the Mac, Lots of good reviews, personally I was unable to get it to run correctly. Ran out of time to track it down.