Same way as you would place ads on any website Edit the template files and add the Adsense code to where you want it to be.
i hv place the code on template overall_header and overall footer files but ads r not showing wat to do nxt?
Go to phpbb.com and ask the same Question there you will get all the help you need and maybe some good Adsense mods.
There are about 2 or 3 official ad management modifications around. PM me if you're too lazy to search
I have a PHPBB website here: http://www.battlefield3forum.net/ i have only edited 2 templates for the current adverts, but you can edit just about any template you like, there are also 3rd party plugins that allow for extra advert placement options, overall PHPBB is much like any other template, if there is a will there is a way. Good luck