If there are no bubbles in the chamber that means its not getting air injected. I haven't used one before (FW
is all i can afford) but I think you need to adjust the air inlet valve or make sure the venturi is clear.
Also from what I understand, the refugium should be close to a sump so the water can be discharged back into the sump then back up into the aquarium. A tee off the incoming water pipe should go into the refugium then the refugium should have some sort of overflow back into the sump.
(I may be wrong but thats pretty much what I've seen)
hope that helps