[StarBuG]

If you want the ad below the message text and only in the first post for guests only find this in postbit_legacy template:

<blockquote class="postcontent restore ">
                            {vb:raw post.message}

and add below:

<vb:if condition="$post['isfirstshown'] AND $show['guest']">
<!-- YOUR ADSENSE CODE HERE
</vb:if>

However this is not tested!
Try it and see if it works