Jerome and all,
The confusion around this "Enable" signal has to do with the fact that the signal does not show up on the pin that the user choses on the GUI page, but rather fixed on pin 17 for the CNC_net_XP plug-in. I took a look at your modified dll and can verify that it inverted the sense of the motor enable signal, but it shows up on pin 17 no matter what you have it set to on the communications configure page.
One should mention that you don't really need this Enable signal in the first place. You can permanently enable the motor divers so that the motors are powered as long you have the power supply turned on, or if you like to move the motors by hand for some reason place a switch so that you can manually disable/enable them. I prefer to have the motors engaged at all times when the power is on, so that there is no chance of me pushing on an axis and loosing the home reference in between cuts.