Open whatever folder you store your video files in. The default location is C:\documents and Settings\"your account name"\Shared. Right click on a file>click Properties>Opens with "change"> choose "Divx player" from the list click ok. That should make Divx the default player. Note this is for "Windows" I think Mac is different.
There's more info in this thread:
http://www.gnutellaforums.com/showth...threadid=34465