You need to edit the "header" template (See the wiki for help on editing templates:
[Wiki: Admin_CP_Templates] (Broken link, head over to docs.mybb.com instead))
Place your advertisement banner code (HTML) under the following, in that template:
<div id="content">
i still don't get, where i need place the ads code? Admin CP?
Admin CP > Templates > Modify/Delete > Your template > Header > Expand > then follow Chris' instructions.
thanks a lot
i didin't see the Expand button before