find in 'FORUMHOME' template:
<table cellpadding="0" cellspacing="2" border="0" width="100%">
<tr valign="bottom">
<td>
<table cellpadding="2" cellspacing="0" border="0">
<tr>
<td><img src="$stylevar[imgdir_statusicon]/forum_new.gif" alt="$vbphrase[contains_new_posts]" border="0" /></td>
<td class="smallfont"> $vbphrase[forum_contains_new_posts]</td>
</tr>
<tr>
<td><img src="$stylevar[imgdir_statusicon]/forum_old.gif" alt="$vbphrase[contains_no_new_posts]" border="0" /></td>
<td class="smallfont"> $vbphrase[forum_contains_no_new_posts]</td>
</tr>
<tr>
<td><img src="$stylevar[imgdir_statusicon]/forum_old_lock.gif" alt="$vbphrase[a_closed_forum]" border="0" /></td>
<td class="smallfont"> $vbphrase[forum_is_closed_for_posting]</td>
</tr>
</table>
</td>
add bellow:
<td>
<!--- ad code here --->
</td>
this should work, may want to add padding or other css elements to make it look nicer.