how do i play an avi movie on my mac

If you just want to play the movie any way you can, you can give this player a try:

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

If you want to play the movie in QuickTime Player, Perian, which rolls several codecs into one package, might work. If not, you can try the codecs at http://www.divx.com/ and http://www.3ivx.com/ and see if one of them will work for you. Do not install both Perian and the DiVX/3iVX codecs at the same time.

Regards.