Instructions
Start the "Mario Kart Wii" game, then move your Wii remote over the "License Setting" option. Press the "A" button on the remote.
Click on the "Change Mii" option. Find a Mii character that you want to add, then click on the Mii by pressing the "A" and "Z" button at the same time.
Move the Mii character to the license and release the buttons to drop the character. Press the "A" button to start the game.
Start the game and select your Mii character. The new Mii character will have a new license. Start playing a new game in any mode.
Choose the character you want to play, then start racing in a game. You'll notice that the kart you're driving has a new license plate with a different number on it than the previous one.