There are many programs for the mac that can play DIVX on the mac.
Here is a history version of DIVX on the Mac
http://www.macopz.com/columns/mpeg4/
www.divx.com also has an alpha codec, but I would avoid that until it goes to beta, at least.
www.3ivx.com has a program called DIVX Doctor II that can convert DIVX files into 3ivx files (3ivx is also another version of the MPEG-4 codec).
http://doctor.3ivx.com/
There is also another older program called "DIVX Player", but you need WMP 6.3 to play it
There are many other program, If you search something like
www.macupdate.com www.versiontracker.com and type in DIVX you will get a whole load of programs to decode it, encoding it is a whole different story.
Anyways the best solution is the Divx doctor, even thought it is a little incovient.
QuickTime 6, delayed because MPEG-LA can't get it out of their minds on their ridiculous licensing fee, will be the new player that will be the official MPEG-4 codec. When this come out, than Real, and WMP will soon follow, and the MPEG-4 will be able to let you share your file with any of the 3 major players (Quicktime, Real, WMP).
Why do I know so much about this, it is because I am always frustrated to tell people to download this codec or to download this player, just to play a video file. MPEG-4 will be the perfect codec that will be a dream come true for all web masters and video professionals. Right now, only cineapak is the only codec that you could find on most machines, but it sucks. Its old, and doesn't produce that great of results.
Anything else, just ask me, I probably know it.