Promo is ending tonight at midnight PST, but you have until the end of the month to actually redeem the promo credit. You must have a Kindle app (or device) linked to your Amazon account to redeem this offer. If you don't have a phone or a tablet, you can download the app for the PC. Just login once on the Kindle app to link.
Short version: When you buy a graphic novel, you get an account credit good for a free digital Marvel graphic novel. It is supposed to require a physical purchase, but it works on digital purchases and it also works on free ($0.00) purchases.
Step 1: Purchase this free Doctor Who comic book:
Step 2: Wait two hours for the promo email to appear in your email.
Step 3: Purchase one of the following 100 Marvel Graphic Novels from the Kindle Store and it will be free after the automatic promo credit: