I'm going to sticky this for the next week or so so people can see this.
For those of you Mac users who purchased BttF without reading the General FAQ or the system specs listed in the Store- you need OSX-6 (Snow Leopard) to play BttF. It's been listed as such from day one.
Somehow, Episodes 1 & 2 of BttF also work on Macs running OSX-5 (Leopard). Episode 3, however, will crash on 5 and run fantastically on 6 (I've tried it- my father's computer is still running OSX-5, and I have an OLDER computer that's running OSX-6. Mine plays BttF glitch-free, his doesn't work at all).
The only thing I can recommend as a fellow gamer is to update your system. If you already have Leopard, Snow Leopard is not an expensive update. It's 29$ by itself, or an additional 100$ for iLife and iWork added.