Brother HL-3450CN - Manual (Page 123)

CHAPTER 5 MAINTENANCE
5-7
4. Insert the new Toner Cartridge(s) by positioning it in the correct guide(s).
Slide it gently into the printer. Do not try to lock or push it in; it should rest
loosely in the guide rails. Note that each color cartridge is individually
keyed to prevent improper installation. Match each cartridge color label to
the corresponding label on the printer.
Fig. 5-2
5. Close the Front Cover.