As far as I know all dot-mac addresses were migrated to dot-me domains a few months ago. Log in to iCloud in System Preferences > iCloud. You need to be running OS X 10.7.x "Lion" or later to be able to use iCloud.
FYI a "MAC address" has nothing to do with a Mac (computer) and is unrelated to an email address. Confusing? Yes, but knowing this may help you find a solution.
Can I still use my mac.com ID as an Apple ID?
Yes. Your @mac.com ID will still work as an Apple ID for purchases on the iTunes Store, Apple Online Store, and to purchase Apple print products created with iPhoto or Aperture. You may continue to use your @mac.com ID or your new me.com ID as an Apple ID. Both names will access the same account.
Can I send and receive email from both @mac.com and @me.com?
If you had a working @mac.com email address as of July 9, 2008 you can continue to use that address as you always have, as well as your new address @me.com. And emails sent to either address will appear in your inbox on the web as well as your devices. For instructions on how to use both email addresses, including how to set up your Mac, PC, iPhone, or iPod touch, read Setting up MobileMe email on your Mac.