EDIT: This is no longer available.
If you have a Twitter account (and judging from the 57,000 of you following HIF, you do!) then use it to get a . After clicking the yellow “Tweet and get $5 button,” it'll link your Twitter and Amazon accounts together and then you should get a confirmation screen saying the credit has been applied to your account.
You can use this credit to “rent a new release movie, catch up on recent TV seasons, or even watch your favorite TV shows the day after they air.” And if you want to unlink your accounts, well then check out this handy guide I wrote a while back.