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]
cscgal
The Queen of DaniWeb
19,420 posts since Feb 2002
Reputation Points: 1,474
Solved Threads: 229
No. It would be
<if condition="$post[postcount]==1 || $post[postcount]==11 || $post[postcount]==21">
cscgal
The Queen of DaniWeb
19,420 posts since Feb 2002
Reputation Points: 1,474
Solved Threads: 229
cscgal
The Queen of DaniWeb
19,420 posts since Feb 2002
Reputation Points: 1,474
Solved Threads: 229
The above code would work for the first three pages - the ad would show after the first post (page 1), after the eleventh post (page 2), and after the twenty-first post (page 3). You might be able to use some modular (remainder) division to make it work for an infinite number of pages. I'm not so sure if the vBulletin templating system supports mod division (%), but it's worth a shot ...
<if condition=="$post[postcount] % 10==1">advertising here</if>
Basically, this means to show the ad code if when you divide the number of the current post by ten, you get a remainder of one. In other words, when you do the division 1 divided by 10, you get 0 remainder 1, so you show the ad. Similarily, when you do 21 divided by 10, you get 1 remainder 1, so again show the ad. You see how this will work for an infinite number of pages.
To answer your second question, there is a ten post minimum on DaniWeb before you have access to forum signatures. Only two more posts to go, 007!
cscgal
The Queen of DaniWeb
19,420 posts since Feb 2002
Reputation Points: 1,474
Solved Threads: 229
To display them to guests only just do
<if condition=="$post[postcount]==1 AND !$bbuserinfo[userid]">
cscgal
The Queen of DaniWeb
19,420 posts since Feb 2002
Reputation Points: 1,474
Solved Threads: 229
In the old version of DaniWeb (pre-April) we had some areas of the site reserved for AdSense, such as below the first and last posts of a thread. However, AdSense is now rotated with the other ad spots :)
cscgal
The Queen of DaniWeb
19,420 posts since Feb 2002
Reputation Points: 1,474
Solved Threads: 229