In Stage 2, while the Behemoth is chasing Richter, there is a candlestick that contains the "key" subweapon. Make sure to get it, and then proceed down into the sewers after the Behemoth is blocked by the wall. At the very end of the sewer path, there is a locked door. Use the key subweapon and it will unlock, giving you a cinematic scene where Maria joins Richter. She is now selectable as a playable character in the main menu.