If you meant where the ad shows after the first post in a thread then you ad the adsense code to your post bit leagacy in Post Bit Templates.. like here..<!-- / post #$post[postid] -->
<if condition="!$GLOBALS['FIRSTPOSTID']">
$spacer_open
<div style="padding:0px 0px $stylevar[cellpadding]px 0px">
<table class="tborder" cellpadding="$stylevar[cellpadding]" cellspacing="$stylevar[cellspacing]" border="0" width="100%" align="center">
<tr>
<td class="thead" align="left">Sponsored Links</td>
</tr>
<tr>
<td class="alt1" align="center">
<center><script type="text/javascript"><!--
google_ad_client = "pub-1343363519719067";
google_ad_width = 728;
google_ad_height = 90;
google_ad_format = "728x90_as";
google_ad_type = "text_image";
google_ad_channel ="";
google_color_border = "8B814C";
google_color_bg = "FFFFFF";
google_color_link = "940F04";
google_color_text = "940F04";
google_color_url = "940F04";
//--></script>
<script type="text/javascript"
src="http://pagead2.googlesyndication.com/pagead/show_ads.js">
</script></center>
</if>