Okay, lets give this a shot then. In Display.template.php,
<div class="inner" id="msg_', $message['id'], '"', '>', $message['body'], '</div>
' . ($message['counter'] == 0 && $context['user']['is_guest'] ? '<br /><div style="text-align: left;">
YOUR AD CODE HERE
</div>' : '') . '
All you have to do is replace YOUR AD CODE HERE with the code snippet you provided.
Therefore, the entire code modified should look something like,
echo '
<div class="inner" id="msg_', $message['id'], '"', '>', $message['body'], '</div>
' . ($message['counter'] == 0 && $context['user']['is_guest'] ? '<br /><div style="text-align: left;">
YOUR AD CODE HERE
</div>' : '') . '
</div>';