In Super Smash Bros. for Nintendo 3DS, you can visit the in-game AOC Shop to purchase add-on content for the game using your Nintendo eShop funds. You can increase the number of playable fighters and Mii Fighter costumes by purchasing add-on content.
Super Smash Bros. for Nintendo 3DS
There are two options for purchasing add-on content: 3DS, and 3DS & Wii U.
The Wii U download code will be listed on your receipt. You can view it by opening Nintendo eShop on Nintendo 3DS and selecting Menu → Settings / Other → Account Activity, then finding your AOC purchase and selecting View Receipt.
Select Enter Download Code from the menu at the top, 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 Wii U.
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.