Well, it depends on where youw ant them... The only "stock" way of putting an ad on "every" page is one of the two sidebars... There is an ad island for under the first post of every page in a thread which can be obtained through editing each forum's settings.
Now if you wanted ads somewhere other than those two places, you'd have to edit templates to fine tune the placement.
It's my hope that sometime in the future we'll have an "Ad Island" feature that would allow you to insert ad islands outside of the sidebars and under the first post...
