The credit gets loaded into whatever account you were logged into when you redeemed. You can then use that credit as long as you are logged into that account when you buy items from any of the 'Stores' (whether it's on your iDevice, iTunes, Mac App Store, etc).
Specifically, in the Mac App Store, you can find the links you want on the right side, below the 'feature' box.
I find it interesting that there are some gift cards that are labeled to be specific for apps, music, books, etc when it really doesn't work that way. It pretty much all goes into your account and you can buy whatever you want with that money in your account. However, I actually have never purchased or known anyone that have purchased a gift card that had a specific digital download of an album with a dollar value. I assume the code automatically starts the download of the album like the digital copy of the videos before that stupid, ridiculous ultraviolet.