In update 1.5.0 we'll be adding the ability to play against your friends without being connected to Facebook. This update is slowly being sent out to our players. Due to this, some players may not have the friend code or area to enter the code. Their games are therefore incompatible with other versions. Don't worry! The update will be with you soon. Here's the old multiplayer guide
Here's a new user.
To start a game with a friend select New Game. You will be presented with this screen: