Been a while since I set up MACH3 and have since switched to EMC but unless you have set up a relay you will need to manually turn your spindle motor on and off. See the G540 manual on connecting a relay. Download the correct version from Geckodrive.
In Mach3 configure Output Signals Tab on the Ports and Pins Dialog and uncheck the Disable Spindle Relays in spindle setup.
hope this helps