I have a WiiU as well. As I recall video should only appear on the game pad at the initial start up until you initiate the set-up. Not to insult your know-how, but did you press 'continue' or something of that sort on the WiiU game pad?
If not hook the WiiU directly up to your tv using the same cable and try to isolate the problem.
I hooked it up both ways, first to the TV itself via HDMI, got only video, no audio. Then to the 4306 and didn't get anything, no video or audio. All my components are thorough the receiver, then to the TV, Pioneer Elite.
Perhaps it could be a handshake problem with HDMI? WiiU output is HDMI 1.4, have you updated the firmware of your AVR-4306? I know new versions of HDMI are supposed to be backwards compatible however there may be an issue here if you haven't updated you firmware in a while. Are you using any other devices connected to your receiver with HDMI beside the WiiU?
Ah, that might be it... The firmware has never been updated. Everything is HDMI, including the DirectTv, which is a new receiver, but I'm sure its an older version. The Wii-U is the only thing that does not work... I will try and update it and try again...