To use a WAD file, your Wii must be "softmodded" with the Homebrew Channel.
If you are looking for a fresh experience, the modding community created . This is a massive, fan-made sequel with 128 new levels. While often distributed as a patch for your existing game, it represents the pinnacle of what the Wii modding scene can do with the original game's engine.
Open your WAD manager via the Homebrew Channel, select the file, and choose "Install."