Scratchbox requires Linux as its host OS, so you'd need to install Linux on your Mac (I recommend http://refit.sf.net/ for dual booting). Alternatively, you can run Linux inside a VM. As far as I know, and from my understanding of how scratchbox works, it's not possible to run it on Mac OS X and won't be anytime soon.

I've installed scratchbox on my Linux machines and I can use it fine over remote X from my Mac. Maybe that's a possibility for you too.