To answer your question, the email importer works with free accounts and I just tested it and it was working for me, but why not use the iCal importer instead? This will do get all your RTM tasks imported in one step. Or, if you'd rather script it, why not try using our API?
Make sure that you have enabled email importing (make sure the checkbox is checked) and if that doesn't work, try changing your secret word. If that doesnt work, make sure that your email program is sending the emails as "plain text"