The easiest way would be in here.... adding a simple text but everyone would read it when they message each other. Which is not so bad either. Please do not contact author for support, use article system blah blah
// Main message editing box.
echo '
<div class="cat_bar">
<h3 class="catbg">
<span class="ie6_header floatleft"><img src="', $settings['images_url'], '/icons/im_newmsg.gif" alt="', $txt['new_message'], '" title="', $txt['new_message'], '" /> ', $txt['new_message'], '</span>
</h3>
<div>Some warning text can be placed here</div>
</div>';