Use the code AVODGIFT when you're renting the movie.
Details on how to use the code.:
Sign up for an Amazon account if you don't already have one.
Enter the code AVODGIFT
Shop for a video in Amazon's video on demand section.
Use your code by Jan 3rd.
You won't instantly see the code being applied when you buy or rent the video, but you can verify you were charged $0 by checking your order confirmation.
We used this to watch a free movie last night and it worked just fine for us.