I've bought Plants vs Zombies from Steam and when I reach an certain point in the game where I obtain a key the game crashes? Any ideas?
This is currently a known issue in the Plants vs Zombies Mac client through Steam. Everyone (or at least a lot of people) are crashing right after receiving Crazy Dave's key. Large numbers of people have been complaining to PopCap to fix it for a few weeks now, and no response has arrived.
The only thing you can do right now is wait it out. Sorry! Keep your eye out for a patch notice through Steam.