Hi fellow Mac users,
the problem is that the mplayer builds supplied by the mplayer people themselves are really outdated so it is suggested to stick with the "unoffical" builds or compile the source from the Subversion repository. Since compiling is usually painful, unless you already have set up your development enviroment for another reaseon, we will stick with the unoffical packages, which you can grab from here:
enjoy!
Update 1:
there is projected called mplayer extended which has nice improvements for the UI and also comes with very recent mplayer builds, including multi core support (FFMPEG), etc. check it out here:
mplayer is now available from the Mac Appstore as well. Just search from mplayer and you will find it.