Here's the solution I came up with for playing MSS2 files. Intel Macbook. OS10.5.6.
I have mplayer intalled via macports.
http://www.macports.org/
Then grab the Windows-All dlls from the codecs directory on Mplayer's site.
http://www.mplayerhq.hu/MPlayer/releases/codecs/
Download and unzip. Copy the contents of the windows-all-xxxxxxxx folder to the macports directory: /opt/local/lib/codecs/
You may have to make the `codecs' directory.
Now I suspect the only dll that matters is wmsdmod.dll. So You can copy that one _only_ and it may play just fine.
I'm still having trouble with the colourspace tho. You'll see what I mean when you play a video. If anyone can figure out how to fix this, please let me know!