Don’t confuse Avast 4.1 with 4.5. They have a different way of scanning mail. I suggest you upgrade to 4.5 beta (final is scheduled for release next saterday anyway) and only look/search on how to set that one up.
I think the best you can do is download the version 4.5 Beta because it uses a new mail detection that will work automatically. Please, set up your email account just as avast does not exists. It will detect the pop3/smpt/imap ports and scan your messages.