This is my code, not only do you get a google ad in there, but you can also direct your users to sign up for a paid subscription!
Code:
<!-- / random ad code, in to add usergroups add numbers to array(1) -->
<if condition="is_member_of($bbuserinfo,array(1,2,6))">
<if condition="$GLOBALS['adsense_position'] == $post['postcount'] && $GLOBALS['adsense_position'] > 0">
$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">
<!-- ad code goes here -->
</script>
<p><center>Would you like to get rid of these ads? Then purchase a higher level of
membership! Click <b><font size="4">
<a href="http://www.yourwebsite.com/forum/payments.php">here</a></font></b>
to find a membership that will work for you!</center></p>
</td>
</tr>
</table>
</div>
$spacer_close
</if>
</if>