Can't be done with a droplet in settings.
You need extra code wich mail a warning when and only when a submission is made, therefore the best option is to add that code to submit_comment.php,
search for: around line 121,
// Insert the comment into db
A few lines later, right before the header statement this code can be added.
// Sending the email
$mail_to = 'firstname.lastname@example.org';
$mail_subject = 'A comment has been made';
$mail_message = 'One newsitem has been commented on this page: '.$wb->page_link($page['link']).'?post_id='.$post_id.' !';
Didn't test it!! but think it'll get you started