you must have downloaded the demo by accident! it has a completely different storefront and set of downloads. the finished version of the game (the one you're on right now) never had a mac version. have you tried playing it in your browser instead?
aw heck, that's been happening to a lot of people. try google chrome and firefox, and if neither work, you might need to enable webgl. a quick google search will give u tutorials on that. sorry i couldn't help much more
