The Wii U is Nintendo's newest console, replacing it's predecessor the Wii.
While the Wii U itself has no available hack yet, it does offer backwards compatibility with the Wii. This compatibility is provided through the "Virtual Wii Mode", which can be accessed from the Wii U main menu.
The virtual Wii Mode is not entirely the same as a normal Wii. A lot of patches were added, and older softmodding processes do not work.
It is possible to softmod the virtual Wii however. The Exploit used to do so is Smash Stack.
While the virtual Wii removes smash stack automatically when it is found on the SD card, the exploit can be used by inserting the SD card AFTER starting Super Smash Bros. Brawl.
As such, you can still use Smash Stack to install the Homebrew Channel, which has been updated to support the virtual Wii.