Remember, you can register Mario Kart 8 today for a free Wii U game from the select list, but it appears as if there's a massive amount of traffic on the site at the moment, and the login servers have been hit pretty hard.
I've been experiencing a number of outages for the past few hours, and many others report that they can't redeem their code. If you can get in, I'd wait a little while for things to cool down before you plug in your registration code -- just in case.
It's as if millions of voices cried out "give me my free Wii U game!" and were suddenly silenced.