You really don't need a mod for this.
vBulletin has a built in "Ad Location" template called
ad_showthread_firstpost_start.
Simply go to your style manager, edit templates, and open that template.
Paste in your ad code from google directly into that template.
To make it visible only to guests add this template conditional:
Code:
<vb:if condition="$show['guest']">
all ad code here...
</vb:if>