not trying to be mean but if you cant assemble the charcoal container then you do not need to be keeping saltwater fish.
but anyways, there is a black rod that snaps in place on the inside to keep the black top and bottom from separating from the mesh screen. geat the screen in the grooves of the black plastic and snap the rod in place. viola! an assembled media cartrige. its about 4 pieces i think (mesh, plactic skeleton top bottom and back, mesh screen, and cap) and should take about 15 seconds to fully assemble. good luck