Online Key? (retail)

So I've been having troubles with my Far Cry 2, and it's really bugging me. I finally installed my copy today, and I've been trying to figure out a way to play multiplayer with a friend. I've read several theorys, such as it really being just the CD key, or that I have to register it on Ubisoft's site, or that I have to enter the key using the numbers on my numpad. And I just want to play some multiplayer. So could I have some help.