Just go there, Like it, click the giant "Get App Now" thing (or do that here if you already Liked the page before), allow it to interact with your Facebook profile, and then click the Redeem button. Then you copy/paste that code into iTunes's redeem page. If you don't know where that is, check the bottom of the Store page or the drop-down menu next to your login email. Pfew!
[*].disqus.comto your security software's whitelist.