Joel - make sure the user selects their camera in the Meeting
menu > Select camera. Mac users usually see a number of options
in the drop down, so they may need to try each one before it
turns-on within the room. I've probably helped 10+ people with
these instructions and it all worked-out.
In saying that, I've seen more advanced and complicated
issues with Macs and cameras on this board, so you may want to find
those threads.