If you are just needing a 'Spindle On' signal from Mach to drive a relay then 'Step and Direction' is entirely the wrong thing to be setting (makes me wonder if the guy you bought you BoB from knows anythign about Mach). Turn off your spindle related stuff in ports and pins and then click on the 'Spindle Setup' tab, look in the top left corner in the section which is coincidently labeled "Relay Control". This is where you set an Mach Output signal that is used for the relay. What is an output signal you ask?
Click on the 'Output Signals' tab, here you will see a list of signals within Mach that you can map to (attach to) a pin on your LPT. If you are not using any of these outputs, just set 'Output #1' to the correct port and pin for your relay and then enable it. Now go back to the 'Spindle Setup' tab and make sure that both CW and CCW (Anti-clockwise for you Brits) are set to Output #1 and that 'Disable Spindle Relays' is NOT checked.