There is a thread for MKWii codes. It is found in the main MKWii forum, and it is pinned to the top. It says you are supposed to pm your code to Jules and he will put it up on the list.

__________________
0173-0974-1865 < Brawl friendcode!4253-4013-1847 < MKWii FC!
{Spur of the moment Toad Mario Kart poem below! lol it is bad!}