Sorry you haven't gotten any responses, here....let's see....
If I understand correctly, there are two black plastic "boxes" full of holes (mesh) that rest in the back of the filter (one on each side)...is that correct?
If so, they are just for extra media. You can put whatever you want in them...floss, carbon, ceramic pieces, even Peat, whatever you think your tank needs. You DO need the bio bags for that type of filter, and they should slide into slots in from of the boxes. The boxes can remain empty and even removed if you don't want to use them.
Hope that helps