It's actually pretty simple. If you just want to add Google Adsense then sign up and generate the appropriate code and paste it in your board. Usually in the 'overall_header.tpl' or 'overall_footer.tpl' which you'll find in whichever template you're using for your board.
If you want to serve a static ad you can just paste it directly in one of those two files but if you want to get a bit fancier you can suse an ad serving program like phpAdsNew (my favorite) which will serve your ads up dynamically.
Hope that helps.
Lux
PS: If you need more help with this hit me up and I'm sure we can sort something out.
Phpadsnew is complicated to use and instead I reccomend using Nkads (http://nkads.nkstudios.net/english/index.php) and also if you want to add the ads it has to be done in /templatedirectory/indexbody.tpl