via
GameSpot
Maybe Don't Bother With That GameCube Controller Adapter; It's Only Compatible With Super Smash Bros
According to GameSpot, Nintendo has specified that the GameCube controller adapter will only work with Smash:
"The GameCube Controller Adapter for Wii U is compatible with Wii U and Super Smash Bros. for Wii U specifically - it is not compatible with any other Wii U software. The adapter is also not compatible with Wii and we apologise for any confusion."
"The GameCube Controller Adapter for Wii U is compatible with Wii U and Super Smash Bros. for Wii U specifically - it is not compatible with any other Wii U software. The adapter is also not compatible with Wii and we apologise for any confusion."