You're going to want to 'post' a message to (what I assume is) the iframed ad telling it how far you have scrolled. If your ad is not iframed, then you can use a Viewport Action.
To get the scroll position of the page and feed it into the iframed ad, you would need to run a script in both the parent page and the ad that talks about how far the person has scrolled. This thread should be helpful: Jquery Waypoints scroll animations in iframes don't work. If you are only allowed to display a single page in an iframe, you won't ever know the scroll position of the page unfortunately.