We've just integrated support for ads into our free video plugin for WordPress.
Make sure you update to the latest version - 2.1.19, and then follow the instructions on our How to use ads in FV Flowplayer page.
You can either insert the ad code (usually a SCRIPT tag), or write your own HTML. Programmers can use WordPress filter hook
fv_flowplayer_ad_html. Detailed information posted in the link above.