Forward request to port 5903 to port 5900

On my tomato router, I could have any incoming request on port 5903 direct to port 5900 internally. That way I could set my external VNC port to something like 5903 but it would still forward to port 5900 internally. Doing so would allow me to have unlimited machines running vnc on port 5900.

When I try to do that on the quantum router its not working. What is the method to this? What am I doing wrong? What is the bottom option for? 

THANKS!!!!