Did you migrate your MobileMe to iCloud before I August?
If you did, you can sign in with the @me.com address and make use of its email and syncing, or, you can sign out and ignore it.
If you didn't, you can sign into System Preferences>iCloud with the @me.com address to reactivate it but you cannot retrieve the previous contents of the account.
Either way, you can sign out of it and sign into System Preferences>iCloud with another Apple ID, whereupon you will be asked to create a new email address: this will be an @icloud.com address and you cannot use the part before the @ of your MobileMe email address (though you can add that in Mail, Contacts and Calendars to use both of them).
You should go to http://appleid.apple.com , sign in with your ID, and ensure that there is a functional email address associated with it so that you can retrieve verification emails.