Scroll to the bottom and tap on Redeem along the bottom.
Enter Tap You can also enter your code manually, enter the code we sent you, and tap on Redeem.
You should now be all set.
If the app you want to redeem does not automatically download tap on Purchased along the bottom of the screen and the app should appear in the purchased list with a little cloud icon next to it. Tap on the cloud icon and the app will begin downloading to your device. More details in regards to this are available on Apple's Support site at http://support.apple.com/kb/HT2519.