I did answer this before but for some reason my post has gone, you can use the built in ad_location templates to post your ad, if you dont want to use those then edit the postbit template and ad this above $message
HTML Code:
<if condition="$post['postid'] == 1">
Your ad code here, you could add a class and use CSS for its look and positioning
</if>
, you would be better off using the ad_location templates