Sounds like it uses some form of the DivX codec.
You can usually check what compression format was used by selecting Movie-Get Movie Properties from QT Player. Then Choose Video Track and Format from the Properties window.
Anyway, to get a Mac DivX player go to:
http://mac.divx.st/
You also need Windows Media Player *6.3* for the DivX Player to work. According to the DivX site MS has pulled it from distribution...
Of course it's always possible that your file just won't play on the Mac period. Not all variants of DivX are available for the Mac...
-Tee