If you ever try to play certain movies on your Mac without having the correct driver installed, you are going to get nothing more than a black screen. No fear! Perian is here to save you! Just download and install this little applet which adds itself to your System Preferences menu, and you can watch all those movies you weren’t previously able to view.
If that alone doesn’t do it for you, it’s possible you need to install the DivX codec and maybe even the XviD codec. If anyone knows of other drivers that help play media on the Mac, please drop a link in the comments!