I would be surprised if it showed in VST connections because that’d mean it could stream audio as well (which it can’t as that’d be pointless 
).
If you set the rock band controller as input on a midi track and hit some pads, does the midi activity bar in Cubase spike?
If not, then you’ll probably need something like Miki posted (if it exists for Mac) because the controller doesn’t send data according to the midi spec.
If you do see activity, it’ll be easier, then it’s a matter of filtering the information out. Try recording some hits on the drumkit to midi (doesn’t matter if you don’t hear anything). Now open the recorded midi part in the list editor and see what’s there.