Octopus BH-100F Overflow box question

I got an Octopus BH-100F Skimmer in today.

It comes with an overflow box... I don't really get what it is for... as far as I can tell, the whole overflow box falls well below my water line.

It's submerged, not overflowing. What should I do? Am I suppose to somehow mount the whole HOB skimmer higher up?

 

Its the surface skimmer box. Its really not a "drop-in" thing because of the different lip designs of the different tanks (on mine it is perfect). But if you want it to fit perfectly, the adjustment/mod is easy. The input neck pvc pipe of the skimmer should have 4 grooves, and one o-ring. You can take a hack saw and cut the pipe at one of the grooves (whichever one suits your waterline the best). Leave at least the top groove for the o-ring. Move the o-ring up to the appropriate groove, and whalla! Perfect fit ;D