"warning this file contains an unknown track format you may need to install a Quicktime decoder"

Hi this message pops up sometimes when I’m trying to watch a movie. I tried installing the Quicktime Decoder, but since I’m on a Mac (OSX Snow Leopard) it says I have a newer version installed. I have the newest version of DivX but I have no Codecs installed.

I’m new to this and would appreciate any help thank you :)