There are a couple of situations I can think of when you need to redeem a code on iTunes. Whether it is a prepaid scratch card or an app promocode, the process to redeem them is the same. Now that many of us are shifting towards post-PC living we need to find a way to get around this without having to use our desktop computer using iPhones and iPads directly.
While the redeem option is something very prominent on iTunes for PC and Mac, on iOS devices this isn't located under the account settings. It can get quite messy if you go access the Apple ID either on an iPhone or iPad, so let's try to stay away from it. The method I'm using here works under iOS 5.1 but since the App Store changes tend to be independent from main software releases, this should work with previous and future versions too.
To redeem your promotional code or gift voucher simply launch the App Store app and navigate to the first bottom tab called 'Featured'. Apple tends to update this section fairly often but don't bother with the banners at the top or the 'What's Hot' category — as long as you're on the 'Featured' tab. Regardless of where you are on the page, scroll all the way to the bottom. Over there, just above your Apple ID you'll see a button clearly labelled 'Redeem'.
Tapping on the 'Redeem' button opens a dialog box where you're meant to type in the code. A word of advice here: since iOS supports copy and pasting, this is a perfect occasion to use it and avoid mistakes. If you have the code written down on an email for example, you can switch between Mail and App Store apps double-clicking the home button. If you have a physical scratch card, you can also think of typing it somewhere else if you want to and follow the same process. This have the advantage of using the landscape keyboard, which isn't supported on that redeem window.
A quick and convenient option
Since I write about iOS apps, I tend to receive many of these via the contact form. Promotional codes seem to be the standard way to distribute apps to the press and beta testers before the app is available for the general public on the App Store. The main reason why I redeem these codes on my iPhone and iPad and not in the desktop isn't simply for convenience. Download codes are produced in batches and have an expiry date — meaning they must be redeemed before a certain time. Also, the codes could have been redeemed already by a different user or contain a different app, you only know once you enter the code and your iTunes password. By redeeming these codes as soon as I get them I avoid many confusions and don't need to keep remembering to do it later before they expire.