Some of this you may have done - but I will have to start at the begining.
Your machine is controlled by a computer, via the printer cable, which has 25 wires in it. For interest the printer wire is actually three addresses in the computer. One address - pins 2 - 9 - are outputs, normally (but not necessarily) used to control the axis of your mill. The second address - pins 1,14, 16 and 17 - are also outputs and control such functions as spindle motor, coolant etc. The third address - pins 10 - 13 and 15 - are input wires which take information in to the computer.
Mach 3 can be configured to use these pins for various functions.
Your spindle is configured thus - go to spindle set up, untick disable spindle relays and enter an output pin number for M3 and M4. Go back to outputs and under those output numbers, enter the pin numbers that are connected to your spindle control system ( If you have one fitted to the mill)
If your machine is set up, then M3 should start the spindle clockwise, M4 starts it counter clockwise and M5 turns it off.