What forum software are you using? Whatever it is, you should be able to go to the forum of that particular homepage and already find the information you are looking for.
and try to put it in a module (even code a container for it yourself) then include it from your main code, that way if you decide you want to change the ad then you don't have to dig around a bit of code that has 500 lines in it just to find the google ad - you just edit the 'google-ad.php - or hatever language its in =)