Mine appears near the top of the Home page of my AdSense account. It says, in a big red bar that goes right across the page: "You have rejected ad requests, which will result in lost revenue. See details."
It's right above a yellow bar message informing me "Your ads have recently appeared on websites you haven't authorized." etc.
Oh, and there's a red "1 Alerts" box on the top right. You can't miss it.