Q: I cannot find where to redeem a digital gift card

You shoudl be able to redeem it on the right-hand side of the iTunes store home page on your computer's iTunes (there should be a Redeem link under the Quicklinks section). Or you can do it on an iPad, iPhone or iPod Touch via, for example, the bottom of the Featured tab in the App Store app

Redeeming a code : http://support.apple.com/kb/HT1574