The Real-Money Auction House (RMAH) is now available since June 12th. Players might still be wondering how to enable it to make transactions.
Like you, I was in the dark. It's not that hard if you have the requirements to activate the feature.
2. If you have PayPal, go to Settings and select My Payment Options. Once there, on the right side is a blue button: "Add Payment Method". There you can enter your PayPal account email, and a secure page will be launched to login into PayPal to approve the linking with the Battle.net Payment Option. If you don't have PayPal, your credit card will do. Simply click the Balance link on the top-right corner of the Battle.net Account Page to transfer a money amount from your bank into the Battle.net Balance.
BALANCE (Credit Card option)
3. Don't have a Battle.net Authenticator? There are two options: the free Mobile app or the physical authenticator. Get the physical Authenticator (USA / Europe) for $6.50 / £8.99 / EUR 9,99. Or download the free Mobile app here for iPod, iPhone, Blackberry, Google Play, Zune/Windows Phone 7.
4. Have a mobile phone? In order to activate the Real Money Auction House, Blizzard requires you to have this extra layer of security: the Battle.net SMS Protect. All you have to do is go to the battle.net/account/management/ page. There are two options beneath your Account Details, under Security Options. Enable the Battle.net SMS Protect feature. Add your phone number. Enable SMS Text Messages. Afterwards, you will get a text message with a security code. Type that security code in the website and hit enter. Your Battle.net SMS Protect feature is then enabled.
5. Launch Diablo III, switch to the Real-Money Auction House. Any time you wish to add an item for sale, or to buy an item select from the dropdown menu the Paypal, or Battle.net Balance options. Voila.
Blizzard will auto-email to your Battle.net account's email address a receipt with the details. This way you can track whether someone is doing things without your consent -- however, with these security layers, that's highly improbable.