Within mach3 the torch fire is normally controlled by output #1 (although any output can be used).... then on the config drop down menu, select ports and pins, and on the outputs tab, select output 1, and input the port and pin number, then go to the mill tab (2nd last one), and check that m3 controls output 1 (spindle clockwise), then go to the inputs and select THCOK, and select the corresponding port and pin numbers for the inputs used.
Everything should work fine then..... if you have problems, upload your XML file and someone will have a look at it for you as there could be other things that catch you out settings wise, but it would take ages to give you information that has taken years to a mass on Mach and setups and what does what.