hey, im a mac user and after EVERY match, whether single or multiplayer, it always says that it failed to save the replay to autosave. whats up with this

Just hit "f" and save it yourself.

There's probably something wrong with your replay folder, I dunno.

It is probably because there are insufficient permissions in the autosave folder

Read here about the latest mac version which fixes permissions issues:
http://forum.toribash.com/showthread.php?t=28821

Or when the autosave folder doesnt exist at all...(almost no chance but it could be)