You have to create a club Nintendo account on their website. After you do that, there's a pin # in your game case, that you put into your club Nintendo account, and after it's verified, they send you the code.
Once you get the code, go to the eshop menu in your Wii U, and go to the redeem code button and put it in and DL.
Here's the steps for your Wii U that'll make it more clear:
- In the HOME Menu, choose the "Wii Menu" icon.
- In the Wii Menu, tap the "Wii Shop Channel" icon.
- Choose "Start Shopping," then tap "Settings and Features."
- Roll down to "Wii Download Ticket," and choose it.
- Tap the box and use the virtual keypad to put in the Wii Download Ticket number then press "OK."
- Press "Redeem," then "OK," then "Now" to download the game.