Sorry for the extremely late reply (I don't pass through here as often as I'd like) but I did a scrolling last entries plugin which you can see here, but I think you mean something which will show ads.
I think the easiest solution would be to hard code something in one of your tpl files (e.g. header.tpl), and I've successfully used a little script called LGR Quote which is quite versatile (you can use it as an ad rotator).
It's not supported any longer, but you can still download it here.
(n.b. I haven't released the scrolling last entries plugin because of an unresolved issue concerning jquery. The plugin works fine but requires a different version of jquery to that which is provided with FlatPress, and boy does it screw things up in the widgets panel if you try and use both!)
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