Instructions
Adding Recent Players to Friends List
Select "Multiplayer" from the Main Menu.
Select "Nintendo Wi-Fi Connection" and press the "C" button to select the "Friends" menu.
Move the Wii control stick to the right to enter the "Recent Players" menu. Move the control stick up/down and press A to select the desired player.
Select "Invite Friend" and press A. The player will get an invite from you and if he/she accepts, then the player will be added to your friends list.
Adding Friends through Wii Friend Code
Select "Multiplayer" from the Main Menu.
Select "Nintendo Wi-Fi Connection" and press the "C" button to select the "Friends" menu.
Move the Wii control stick to the Left to enter the "Add Friend" menu.
Enter the 16-digit friend code and select "OK." The player will get an invite from you and if he/she accepts, then the player will be added to your friends list.