WOW! So NUSspli came out really good to install games, the installation process and time is quite impressive in the WiiU!By common key we mean what you get when googling "Wii U common key". Not allowed to tell much more according to forum rules.
//EDIT: BTW: NUSspli is able to generate game keys without knowing the common key...
But I have to say that I was looking forward to using this program (the Kii-U-Generator from @dojafoja here for the 3DS since I saw that it has the same source code for both 3ds and wiiu games. So I was hoping to get it to work, but I'm kinda bit lost with the common-key thing for the 3ds.