Hey, dda.
First, you'll need to get a vanilla copy of DF. Install that, then get a copy of DFHack (it comes with TWBT) and install that. Finally, drop in the new raw files and make sure you've ported over the init.txt. If you do that, everything should work just fine and you'll get to keep your old saves if they used vanilla raws.
Oh, and make sure you back up everything before you start!
EDIT:
Oh, fudge. I just realized you said you were an OSX user. Sorry, I don't have any experience with that OS so this method might not work. You might want to ask around in the TWBT thread if this doesn't work.