Brother MFC-9550 Owner's Manual page 178

T R O U B L E S H O O T I N G A N D M A I N T E N A N C E
6
Slide the new toner cartridge into the opening on the right side of the
drum unit. To ensure that the toner cartridge and the drum unit fit
together correctly, make sure that the toner cartridge guide bar is
exactly aligned with the guide slot in the drum unit.
Drum Unit
7
Gently turn the lever on the toner cartridge backward until it stops.
8
Reinstall the drum unit into the machine and close the top cover.
Top Cover
Drum Unit
165