I e-mailed them. This is what they said:
"Thank you for your recent correspondence.
With regards to your query, I am sorry to hear that there is no pin code with your Nintendo Super Smash Bro's Wii U.
Club Nintendo pin codes will only be included with new products and not pre owned games or consoles. The purchase must have been made within the last 12 months from a region which has a Club Nintendo service.
Most 3DS and Wii U games are supplied with a Club Nintendo code. With Wii and DS games only first party games such as Mario, Pokemon, Professor Layton and Zelda come these codes.
If your product meets all the above requirements then we will require you to send a copy of your proof of purchase to us so we can look into this further.
If the product was purchased online your proof of purchase must show full dispatch and order details, including all names and addresses. It must also show the VAT registration number of the company you have made the order with.
We apologise for any inconvenience which may have been caused. If you have any other queries or require any other support then please do not hesitate to include this in your reply and we will be happy to help.
Kind regards,
Kafi Gosling
Your Nintendo UK Team
0345 60 50 247"