You don't really need a LAN connection for this, you can unlock it by yourself.
After you enter the match you're in some kind of lobby/free roam where you have to wait for other player(s). However, if you press the ESC key a menu will pop up, giving you the possibility to choose one of the three race modes. Just choose the one you feel the more comfortable with and start playing! Don't mind the "waiting for player" message, achievement will unlock after 10 races :)