Hi - I'm happy to help with your Mac issue. Please remember to Accept when we are completely finished and you are satisfied with my answer. Do you have an Apple ID? Usually you register an AppleID with the iTunes store when you set up your credit card, etc. If you have not set up an Apple ID or iTubes Account, you will need to go there with iTunes and set one up.
Here is a link to to the article Apple published about how to setup and/or change your iTunes Store Account. http://support.apple.com/kb/ht1911