Is the redeem code on the back still valid? Can you re-enter it? What registration information are you referring to? Your Apple ID? When I redeem on an authorized machine, I just go to the iTunes Store home page, select redeem and enter my code.
Never gotten one before (used to think they were idiotic - I mean, who pays for music? hehe j/k).
Think I may get a few for my birthday coming up soon. If that's the case, how do I use one to purchase apps? Usually when you click on it, it just automatically buys it with your credit card on file. I'm sure I could figure it out if I just looked at iTunes, but I'm not home right now.
It's easy... just go to your account in iTunes>>>Click Edit Billing Info and from there you'll see a Gift Cards and Certificates link. Click that, enter the info from the GC and whenever you make a purchase it will deduct the total from your GC before charging your CC on file.
I recently started using them after I got one from my Birthday and I love them. From your phone go to iTunes > More > Redeem > Input the code > Redeem. The credit will then show up on your account in iTunes on your computer as well as one your phone. If you have a credit card on file, it will use up the credit before starting to charge the card.