Oki C 5300n - Manual (Page 136)

C5300 Replacing Consumables • 136
8. Lower the right end of the cartridge into the image drum unit,
pressing it firmly down in place, then push the colored lock lever
back to open the toner slot and lock it in place.
Important!
The lever should move easily. If it does not, STOP! and push
down on the toner cartridge to be sure that it is properly seated
in the image drum, then press back on the lever again.
To avoid toner spillage, do NOT open the toner release lever until
the toner cartridge is firmly seated in the image drum.