You didn't intend to make a new thread did you?
If you can get them all online then you should be able to get them in single player.
Unlock is when you click unlock and type your username and password. After you've done that it'll say "S2 Licenced" on the main menu.
To get the cars in single player you click CAR on the setup screen, and to change tracks you click Tracks in the race setup screen. There's no trick to it, it should just work. If it doesn't try redownloading LFS (I know it's a bummer, but it'll probably fix it).
Welcome to LFS by the way. People here will post saying you'll have no social life now, but just ignore them, because LFS is very sociable
Edit: Gah, beaten to it