I got Aliens vs Predator last night and played single player for a while and nearly had a heart attack, so I moved to multiplayer. I was disappointed to find that I could only play as human. Is there something that I have to do in order to unlock either Alien or Predator teams or will it unlock automatically after a while.
The races you get to play are determined by the game-mode. If you've only played human so far, I think you either played Infestation or Predator Hunt, in both of which you start as a human. You might have also got unlucky and be placed in the human team every time.
In the other game-modes it's Aliens vs. Predators vs. Humans, so you'll have to be lucky and end up on the right team, or you can play Mixed Deathmatch, which has teams with mixed species.