Select the "Special Games" option at the main menu, then choose the "Ring Tournament" selection. Enter "A3W5KQA3C" as a code at the code entry screen to play a tournament for the Mario Cup. Another code will be displayed after completing the tournament. The new code is not actually a code for the game, but is for ranking you with other players at Nintendo's Mario Tennis web site.

Enter "J6M9PQJ6U" as a code to play as Donkey Kong Jr. vs. Bowser in another contest.