Dell 3010cn - Color Laser Printer Manual page 43

Inserting the cartridge
a. Guide the cartridge until it snaps into place.
CAUTION: Do not do not force the cartridge.
b. Push down the handhold to lock the cartridge in place.
CAUTION: If you find it difficult to push down the handhold, remove the cartridge and insert the cartridge again.
4. If replacing just one toner cartridge, close the front door and you are finished.
NOTE: Make sure the toner cartridge is installed correctly. Otherwise, the front door cannot close firmly.
5. Repeat steps 1 through 4 when inserting the remaining cartridges.