To get your code (doing this from memory at work), go to the Mario Kart Channel option in the game, then go to Friends, and your code should be on the bottom of the screen.
Only played a little bit (finished Mushroom & Flower on 50cc w/ gold trophies), but I like how your Mii's show up as the crowds, as well as in courses (see Coconut Mall - Mii's on the walls, driving those big cars towards the finish/start line).