I have a 3DS, if that makes any difference.
First do this:
https://en-americas-...:t1-q1a1-q2a3-c
And this if you never bothered to set up a NNID for whatever reason (you most likely already did this if you've ever bought anything on the 3DS eShop) :
https://en-americas-...on-nintendo-3ds
Once you're done with all of that, you should be able to add funds to your Nintendo Account through the website and have them show up on the eventual WiiU once you connect it to your NNID: