Are you still trying to use the MobileMe servers? this won't work - MobileMe was closed six months ago. Did you migrate your account to iCloud before August 1st?
If so you can sign in with your @me.com address in System Preferences>iCloud and check Mail in the checklist there to set Mail up to receive what is now your iCloud email (still @me.com, plus an @icloud.com version).
If you did not migrate you can reactivate the address by the same process but you will not be able to access any emails previously stored on the account as these have long since been deleted.