Okay, then you'll want the water to flow down the tube which has the airstone in it. That's how it gets the name "countercurrent". A Google search might turn up some information on that brand.
A Magnum might be able to feed it water. I don't know much about Magnums. If not, any small pressure-rated pump should be a good guess. The skimmer was designed for a certain water flow, and I can't really guess what that rating would be. Maybe experiment if you can't find a manual, etc.?