Did you receive an e-mail after your donation? If so, it usually provides you with a linked MAC address.
You can go here to flesh out (or change) your MAC address and create the link.
Where it asks for Donation, use the number listed under "User Name and Password" in the e-mail. If you were provided with a MAC address in the e-mail, you are good to go with that MAC address.
MAC should be 00:1A:78....... along those lines. Use your donation. Username and log in. And add your own Mac to it depending on what system you use you should find you mac in your settings