Here's how I reset the toner error message on my Brother HL-5200DW printer:
1. Open the front door & leave open
2. Press the back button until "Machine Info" appears on the display
3. Hold the "Cancel" and "Secure" buttons simultaneously
4. Use up/down to select the appropriate toner cartridge
5. "Ok" button
6. Hit the up arrow to perform the reset
7. Close the door and print!
I hope this helps people!