If you're using vBulletin 3, go into the Style Manager for the style you wish to edit. The change will be made in either the postbit or postbit_legacy template, depending upon which one you use. (User info on top vs. user info on left).

The very bottom of the template should say:
[php]<if condition="$show['spacer']"><if condition="!$post['islastshown']"></div></if>$spacer_close</if>
<!-- / post #$post[postid] -->[/php]
Directly above this, add:
[php]<if condition="$post[postcount]==1">
<!-- text advertising -->
Google AdSense here
<!-- / text advertising -->
</if>[/php]
Of course, it can be customized via HTML and CSS as I have done to have a background, etc.

(Toggle Plain Text)

<if condition="$post[postcount]==1 || $post[postcount]==11 || $post[postcount]==21">

<if condition="$post[postcount]==1 || $post[postcount]==11 || $post[postcount]==21">