Go to Messages on the Wii menu, click on the create message icon, and when you open your address book your Wii (16-digit) number should be there, first thing you see.
For some idiotic reason though, there is a different number involved when you play Mario Kart with your friends. This is a different (12-digit) number that I guess is created when you get Mario Kart on the internet the first time. They even more idiotically call both numbers registering friends, or at least it's similar enough to be confusing.
