I had my first taste of facebook video calling sometime this week, although i didn't set-it up myself, my friend initiated a video call while we were chatting. I found out that although my account wasn't yet configured, it would initialize as soon as one of your friend (who has an active video calling set-up on his profile) has initiated it for you.
But, if you want to configure your account and initiate a video call to one of your friends, then you must goto this site and follow the instructions:
http://www.facebook.com/videocalling
Seriously, you need just a couple of clicks for you to activate it. Once you ran the pop-up window and have it finished installing (for the plug-in), then just open up your chat window and initiate a video call to your friend who has a camera.
MAC