Instructions
Know your friend's Wii number. This is a 16-digit code that's essentially the online address for each individual Wii. Your friend can find this number on the first page of his address book.
Make sure your Wii is online. You can use several options for this, including wireless and wired options. Use whichever works for you.
Turn on your Wii and your television. On the main Wii menu, choose the "Message Board" icon. It should be in the bottom-right corner and looks like an envelope.
Open your address book. Select the "Create Message" icon from the Message Board screen. It's the second icon in the bottom-left corner and looks like a piece of paper and a pencil. Click the "Address Book" option to the right of the envelope on the "Create Message" options screen.
Register your friend's Wii number. Click the "Register" button by pressing "A" with the cursor over the button. Choose the "Wii" option on the left of the screen. Use the on-screen keypad to enter the 16-digit code from your friend's Wii.