This plugin does not work if the Site Address and URL Address are different. When they are different the plugin uses the home_url to find the admin-ajax.php file when it should be using the site_url.
You can fix this by replacing this (line 117):
$ajaxurl = home_url( '/wp-admin/admin-ajax.php' );
$ajaxurl = site_url( '/wp-admin/admin-ajax.php' );
Hope this helps, and hopefully this fix can get worked into the next version.