open subtemplates/thread.inc.tpl.

Find:(in web_2.0 theme)

<div class="thread-posting{if $data.$element.new} new{/if}" id="p{$data.$element.id}">

Add above:

{if $data.$element.id==3}
{literal}
<script type="text/javascript"><!--
google_ad_client = "pub-****";
/* 728x90, FreeServ Adsense */
google_ad_slot = "*****";
google_ad_width = 728;
google_ad_height = 90;
//-->
</script>
<script type="text/javascript"
src="http://pagead2.googlesyndication.com/pagead/show_ads.js">
</script>
{/literal}
{/if}

Also did so in thread_linear.inc.tpl...