Not a real helpful post I'm afraid, but on PC, you need a seperate 3rd party screen capture program such as 'fraps' to make a video in .avi or whatever format.
You'll need to find a mac program that does a similar thing: ie captures frames from an OpenGL screen, and saves as .mov or whatever.
Technically, you dont even need to record a demo to do this, but its better to use the demo format as an intermediary, since screengrabbers like fraps will degrade the game performance if you're capturing video while playing.