If you have an @mac.com email address, that would imply you are now on icloud. Is this correct?
Or... did you once have an @mac.com address in mobileMe or the previous service and did not migrate to icloud when the old service was shut down. Is this your situation?
You might use your computer's browser and log into incloud.com using this @mac.com address to see if it works, of course you'll have to remember the password. Have you tried this? What was the result?