First off, please keep questions in the Support forum. This forum is for people providing tips, not asking for help.
Are you referring to the Mail app that ships with OS X? If you want this done automatically, you'd probably have to use a script in Mail's rules, not Hazel, to get it exported out of Mail. You can also set Hazel to watch the Mail downloads folder (if you hit the + button underneath the folder list, the open dialog has a pop-up which includes this). In this case, though, attachments only appear in that folder when you open the attachment manually in Mail.