Instructions
Put "Mario Kart Wii" in your Wii console. Launch the game.
Choose "Select License" from the menu. Click the "Settings" icon.
Select "Mario Kart Channel." Choose "Yes," then select "Install" to download the Mario Kart Channel to your Wii home screen.
Launch the Mario Kart Wii Channel. Choose "Select License."
Choose your player profile and select "Friends."
Write down your 12-digit friend code. Give this code to your friend, and get your friend's code in return.
Select "Register a Friend." Enter your friend's 12-digit code and click "OK."
Launch "Mario Kart Wii." Select "Nintendo WFC" from the menu.
Select "Friends," then choose "Find a Friend." Choose "Meet Up" to join an online friend in his current game.