A/D pumps the shotgun. To load, you must be pumped fully forward. To load, drag shells into the loading port (indicated by the arrow) red-side forward. Only 4 shells can be held in the magazine. You don't want to load the shell backwards. Fully pump backwards, wait a second, and then fully pump forward to move the shell into the firing chamber. You should briefly see the shell move into the chamber. If not, pump fully backwards/fully forwards again until this happens. Hit space to fire, and repeating the pump cycle will eject the spent shell and load the next.