Like I said, "last I checked"...
AVI isn't a Mac-native format. Sure, you can get them to play, but you'll need to have the right QT components installed already, or, a program/helper app like Perian.
Still... if your purpose is to score music to this movie and it's a real gig, I strongly suggest getting a DV copy of the movie from the client and ditch the AVI. (Trust me, you'll thank me in the morning). But even if it's just a casual thing you're doing, a DV movie file is going to play back with the least amount of trouble than other movie file formats or wrappers.