Setting up a MobileMe IMAP should not longer work, as the settings will be wrong.
Go to icloud.com and sign in with your apple id (guess that'll be your @mac address?). Verify you see your mail correctly there. If you do, remove your @mac email from mail menu >> preferences >> accounts. Then, go to system preferences >> icloud, sign in with your apple id and put a check beside 'mail'. Your mail will be automatically setup for you in the mail.app. If you want to send from your @mac address, click the pull down menu beside the "from" field while composing an email and select that address.
You can also set default address to send from in mail menu >> preferences >> composing.
iCloud mail settings are not supposed to be edited manually unless you're setting up it up on a system prior to Lion.