incoming mail server field - unable to type address ... please advise ... thanks

You shouldn't need to enter the server manually. Go to System Preferences>iCloud and uncheck 'Mail', then re-check it. This should set the Mail application up for you. If that doesn't work, sign out completely and sign back in (your synced data will disappear from your Mac when you sign out but will reappear - give it time - when you sign back in).