Have u registered with adsense ??
if no, first register for google adsense through *www.google.com/adsense . U can use ur gmail account details for dat.....First ur website will be crawled by a bot and will be verified.....if it is good u will get an email that ur application has been approved....
After
U can add it to the Overall Header or in viewforum , viewtopic, etc.....Search google for that......
Google
U need full FTP access to edit ur files to integrate adsense....But i guess u are using freebb board so u dont have ftp access. Then i cant help...