I'm not familiar with your particular model, but I know that most BT Paperjets are made by lexmark, so I'm guessing this one is too.
There should be a small hole on the raised part of the top part of the cartridge. Enlarge the hole, and with a syringe, fill from there. If that is a tad confusing, You can remove the coloured lid (CAREFULLY) and inject 15-18ccs of black ink into the visible sponge. best of luck!!