You don't register anything - you unlock once you have bought and paid for LFS (i.e. become S2 Licenced). As you are you load LFS and click on Multiplayer, read the little text saying how bad Demo servers might be as it's free access, choose a server and off you go.
When you buy LFS, which I heartily recommend as you'll not only have bags of fun, but you'll also support the development and improvement of LFS, you get a game password and 3 unlocks. You use your game password and your registered username to unlock LFS, which gives you access to all the cars, all the tracks, and the S2 servers (where the racing is usually clean, fair and fun).
