Step one is to purchase a redemption code usable at the Mac App Store. Get it from Apple's online store's telesales agents: 1-800-MY-APPLE (1-800-692-7753) or Customer Service and Sales Support at 1-800-676-2775. Then, launch the MAS, click on redeem, and follow the instructions.
BTW, do note that no PPC apps will run and that the ill-designed installer deletes itself during installation, so when it finishes DLing, quit it, put a copy in a safe place, and then relaunch it. That saves resources; no wasting of bandwidth, time, and electricity DLing it again if something goes amiss or you have to restore it. Finally, the latest version is 10.7.5 and that's what you'll get at the MAS.