Config, Ports and Pins then Spindle Setup. Make sure yo have the tick out of the Disable relays for Spindle. Set the number for M3 and M4 to the same output number, the number being an output you are not using. Then go to Output page and set the correct port and pin number for the output you have chosen (Port 1 Pin1 from your description above).
M3 or M4 will then start, M5 will stop.
Hood
Have added some screenshots to show.