Oops it will work only in the first topic of the all Forum
-------------------------------------
So Try this code

open subtemplates/thread.inc.tpl.(in all themes)

Find:

<div class="thread-posting{if $data.$element.new} new{/if}" id="p{$data.$element.id}">

Add above:

 
{if $data.$element.pid==0 && $category_name}
{assign var=myid value=$data.$element.id}
{/if}
{if $data.$element.id==($myid+2)}
Your ad here:)
{/if}