Its now possible to display Adsense adverts after the first post in your forum especially in the Core theme
You can try this:
Find in Display.template.php:
<div class="inner" id="msg_', $message['id'], '"', '>', $message['body'], '</div>Then replace the above code with
<div class="inner" id="msg_', $message['id'], '"', '>';
if ($message['id'] == $context['first_message'])
echo '
<div style="float:right; width:270px;">
<script type="text/javascript"><!--
google_ad_client = "pub-xxxxxxxxxxxxxxx";
/* 250x250, created 9/07/11 bforum text */
google_ad_slot = "xxxxxxxxxxxx";
google_ad_width = 250;
google_ad_height = 250;
//-->
</script>
<script type="text/javascript"
src="http://pagead2.googlesyndication.com/pagead/show_ads.js">
</script></div>';
echo '
', $message['body'], '</div>
change xxxxxxx to your Google Adsense Publisher ID
