How to redeem code in the U.S. Storefront?

Do you mean you want to redeem an iTunes gift card in the iTunes Store on your iPad?

Open the iTunes Store app on your iPad and scroll to the bottom. At the bottom center of the page will be a Redeem button.

Are you in the US with a US billing address on your account ? All iTunes gift cards are country-specific, they can only be redeemed and used in their country of issue. If you are but your iPad thinks that you are in a different country then try going to the bottom of the Featured tab in the App Store app on your iPad and tap on your account id, tap on 'View Apple ID' on the popup and log into your account, and then select the Country/Region section and select USA.