Because I bought it ages ago now.
Here is a picture of it in the sump.
From what I read at the time, they seemed to work very efficiently.
This picture illustrates why you're struggling. The skimmer operates by adding air to the stone. As the air rises through the water column it will pull "some" water up with it, but not much because air realy isn't a good pump. Since it's designed to be an internal skimmer, the water level looks like it needs to be higher so that the air can be more effective.