Your best best, and easiest, is to add your advertisement code to the header.php and or footer.php files in the includes/ directory. Then your ads should appear at the top and/or bottom of every page in the forum.
I am a complete newbie at Forums but I stumbled across the header.php and footer.php while trying to fufill another request from my client. I gave the whole lot over to my web designer and he has got it going.
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