I haven't done it, but it seems simple enough. So when you're chatting with someone, at the top of the chat window there will be a little camera sign. Click it. After you click it, a window will pop up asking you to run something, I think. After you click run (or download, or whatever it is, I forgot) just follow the instructions from there. I haven't gotten much farther than run, because I don't trust Facebook to download from the website directly to my computer.
I don't know if you've noticed, but Facebook has a lot of viruses. To be safe, I'd recommend using Skype or Oovoo or MSN or something to webcam with your friends.