You’ve got about two weeks to take advantage of the offer, as it expires on December 31st. That means the credit must be redeemed and used by the end of the month. We presume also, that if you rent a title it must be watched before Dec. 31st deadline as well.
To activate the credit find the email from Amazon and click “Redeem Your Credit.” You will then be directed to the Amazon.com website and need to sign-in if not already. Then, Amazon will direct you towards their digital video section.
There is no indication of which titles are eligible, but it seem most of the library is. Of course, Prime titles are already free to watch but if you want to own it indefinitely most of them are eligible for the credit.
Not a bad time to pick up an older Star Wars title, is it?