In Super Smash Bros. for Wii U you can select the Notices & eShop button to access Nintendo eShop, where you can purchase the add-on content. You can increase the number of playable fighters and Mii Fighter outfits by purchasing add-on content.
Super Smash Bros. for Wii U
Nintendo eShop on Wii U
There are two options for purchasing add-on content: Wii U, and Wii U & 3DS.
Please close the software after purchasing add-on content, as the installation will take place while on the Wii U Menu, or while the console is powered down if Standby Functions have been activated. Once the installation has finished, you'll be able to use that content in Super Smash Bros. for Wii U.
The Nintendo 3DS download code will be listed on your receipt.
You can view it by opening Nintendo eShop on Wii U and selecting Menu → Account Activity, then finding your AOC purchase and selecting View Receipt.
In Super Smash Bros. for Nintendo 3DS, open the AOC Shop and enter the 16-character code listed on your receipt. Your download will begin, and once it's finished, you'll be able to use that content in Super Smash Bros. for Nintendo 3DS.