How to Redeem:
Go to Nintendo's website and make sure you're signed in to your Nintendo Account. There are options to sign in with your associated email address, your Nintendo Network ID, or via a social media account. After that, click here: https://ec.nintendo.com/redeem/#.
Enter your code then select ‘Next.’ You may be asked to reenter your account password. Select the 'Redeem' option.
How to check your balance:
Call: +1 (800)-255-3700
Abridged terms and conditions:
When you enter the card’s activation code on Nintendo eShop or other Nintendo shopping services, the card balance will be associated with your Account and will be non-transferable. There will be no remaining balance on the card.
Card balance may only be used on a single Account.
There is a maximum unused Account balance that may be stored on a single Account, and the card may, as a result, be temporarily unredeemable.
Card balance does not expire, and there are no fees associated with the card. This card is non-reloadable.
Balance only redeemable using a Nintendo Account or Nintendo Network ID (“Account”) on Nintendo eShop and other Nintendo shopping services to purchase games and other content.
Valid only with U.S. Accounts.
Chat & Email: https://en-americas-support.nintendo.com/app/contact