In the postbit template
Under
<!-- / post $post[postid] popup menu -->
Add
<table class="tborder" cellpadding="$stylevar[cellpadding]" cellspacing="$stylevar[cellspacing]" border="0" width="100%" align="center">
<tr>
<td class="alt2">
<div id="post_message_$post[postid]">test</div>
</td>
</table>
If you only want it to appear under a certain post then do
<if condition="$post[postcount] == 4">
<table class="tborder" cellpadding="$stylevar[cellpadding]" cellspacing="$stylevar[cellspacing]" border="0" width="100%" align="center">
<tr>
<td class="alt2">
<div id="post_message_$post[postid]">test</div>
</td>
</table>
</if>
Now it will only appear under the fourth post.