The update mentioned by Iwata at last month’s Nintendo Direct is now available for download in North America.  Here are the update details and check out the link for how todownload it.

UPDATE DETAILS
This is an update for the Mar
io Kart 7 game that will resolve shortcut exploits in the following Online Multiplayer mode courses: Wuhu Loop, Maka Wuhu and Bowser Castle 1. Starting 5/16/12, this update will be required in order to play Online Multiplayer mode. You can still pla
y Single-Player mode or Local Multiplayer mode without installing the update. To get the update, visit the Nintendo eShop*.

How to download Mario Kart 7 Update