I'd start by taking a look at log/development.log and seeing if there's some useful information you can post there. Also, it'd be good to know if the page is actually sending an email before it throws an error. If the mail is being sent, it could be an issue with "redirect_to:" on the mailer page part.
Here's a good tutorial: http://www.banane.com/2009/10/27/radiant-mailer-extension-installation-basics/ Pete
