You can use a condition to make it show every X post, and I wouldn't edit the postbit itself, you'll notice in the example above "$ad_location[ad_showthread_firstpost_start]"
there are built in ad_location templates that you should use, much easier when you go to upgrade.
The thing is that I have an "Articles" forum where posts are not allowed but the threads are long and receive good traffic, therefore I am thinking to show the ads inside the thread not below it.
I created a BBcode and it worked, but the problem is that it's now shown to visitors in the bbcode list and I don't know how to hide it.
Any idea!