There are players available that support most DivX formats on the Mac. If it's an AVI with some less common codec, then you're probably out of luck though.

Have you tried the following?

DivX QT plugin
http://www.divx.com/divx/mac/

VLC
http://www.videolan.org/vlc/

mPlayer
http://mplayerosx.sourceforge.net/

convertToQT
http://homepage.mac.com/cgodefroy/macosxdev/index.html

Windows Media Player
http://www.microsoft.com/windows/windowsmedia/software/Macintosh/osx/default.aspx