Go to your forumdisplay template and enter your ad code in between the following two pieces of code:
Code:
<vb:if condition="$show['stickies']">
<ol id="stickies" class="stickies">
{vb:raw threadbits_sticky}
</ol>
</vb:if>
and
Code:
<vb:if condition="$show['threads']">
<ol id="threads" class="threads">
{vb:raw threadbits}
</ol>
</vb:if>
It also might help if you enter your ad code wrapped in something like this:
Code:
<div class="threadbit">
<p class="nonthread">
<-- AD CODE HERE -->
</p>
</div>