Hi Alfred,
If you haven't already, please be sure to check out the sticky post found here: http://wordpress.org/support/topic/plugin-email-before-download-sticky-getting-faster-support
By the way, are you sure on the version of the Contact Form 7, or it's a typo? The latest version of it is 3.4.2 and it works with our plugin.
As for the error message it can also occur if you pass a wrong CF7 form id into our plugin shortcode that in its basic form should look like:
[email-download download_id="5" contact_form_id="4"]
So please check contact_form_id attribute value.
Thanks for the interest in our plugin.