A new update is coming to the Wii U version of the game that will enable GamePad play.
On April 15 (This coming Monday), a new update will be downloadable for Monster Hunter 3 Ultimate that will do two things:
Firstly, it will enable Cross-region play so that both North American and European players will be able to connect to each other’s games, despite being on different servers.
The second part of the update is important for everyone who wants to take full advantage of the Wii U software. Once the update is installed you will be able to stream your gameplay from your Wii U and onto your GamePad! Now you’ll be able to play Monster Hunter 3 Ultimate on your Wii U -and- watch something else on the TV. Huzzah!
Source. Capcom blog