In order to redeem a Nintendo Wii U Games CD Key you can do it both through Wii U and the Nintendo website:

1, Wii U

Make sure the Wii U is connected to the internet.
Make sure the Wii U is the latest version.
Step by step instructions on how to redeem a download code now: go to the Menu and select the Nintendo eShop icon.
Then click on Redeem Download Code and then click OK.
Enter the code in the field then click OK.

2, Nintendo website

You can also download via Nintendo Website:
Please go to Nintendo website: https://ec.nintendo.com/redeem and create an account or log-in if you already have one.
Then please paste your 16 digit code and click Next.
Now just follow the steps on the screen to complete the process.