Here's how the cartridge looks with the bottom opened.
It has 2 sponges inside. One of them seemed to be a thicker piece than the other.
What I did is that I inverted the cartridge, opened the bottom lid, take out the 2 sponges. Place the thicker sponge inside and push it all the way in. really pack it up with the resin. After you have fill it up, take the cartridge and tap it against the floor, the resin will loosen and level will drop, fill it up again and repeat till not much space is left. Place the thinner sponge in and screw tight the bottom lid.