If you had a MobileMe account with an @mac.com address and migrated to iCloud before the end of August last year then you have an iCloud account which is free, including 5GB of Storage.
If you did not migrate your MobileMe account then you don't have an iCloud account and your @mac.com address will no longer function and cannot be reactivated, though you can use the login to open a new iCloud account.
You can test whether you have an account by going to http://icloud.com and logging in - if you have an account you will see the various applications in it on the landing page.
Full details on iCloud here:
http://rfwilmut.net/icloud