You need to be an Audible subscriber to get credits. Free users will not have them as they will expire if your subscription pause. And if you don't have credit, you can purchase the book with your credit card, and I believe that there will be an option showing when you click the book to buy. Besides, you can download the audiobook offline for playing whether you're a subscriber or not. And you can also save your download Audible files on the computer, mobile devices, or other players by converting them to MP3 or AAC, etc. using AudKit Audible AAX Converter.