The Wii U version of Monster Hunter 3 Ultimate doesn’t feature keyboard support. It’s possible to use an on-screen HUD, but you can’t plug in a USB device and type away.
This could change in the future. At Jump Festa 2013 this past weekend, some of the game’s staffers hinted that Ultimate could be getting keyboard down the line. It’d be rather nice to have this feature available through an update by the time the localized version hits store shelves.