[TalkVirginia]

Ok.. I've been able to replicate the issue and will post a fix shortly.

Edit: I left out the step where you need to add the $ad_location[ad_showthread_below_sig] to the Postbit and Postbit Legacy templates.

1. Open your Admincp and go to :
Styles & Templates => Style Manager => Master Style => Edit Templates

2. Find the following hook location in your Postbit and Postbit Legacy templates:

    $template_hook[postbit_signature_end]

3. Add the following ad location below it:
Note: I did this with the thought that maybe you might want to do some calculation in the signature end hook to be included in your site signature. Just a thought.

     $ad_location[ad_showthread_below_sig]

4. Save your templates

Now it should show up. My apologies for missing this.