Ok look at the eBay auction, if thats the same one you have (there are many variations coming out of China) then you will see lower left corner it says 4 Inputs.
If you scroll further down you will see a representation of E-Stop etc connecting to these inputs. It tells you there which pin on the connector corresponds to the parallel port pin so for example pin 1 on the connector on the board connects to pin 10 in Mach, pin 2 to 11 etc. If you have a spare pin then you want to connect it into that then tell Mach which pin you have connected the index to.
Hood