G540 Pin 7 common gound?

I got my G540 all wired up, runs fine with Mach 3 and drives all the motors correctly on the workbench. No smoke, no sparks, no flames, a successful fabrication. I want to install a C15 - Duel Relay Board from CNC4PC.com. The C15 instructions have a warning that "To keep the signals optoisolated, these must not have common ground with the external power supply used to power the board". Will use Pin 5 as the input but need a ground. Pin 12 is a gnd but it is shared with the power supply. I am wondering if I can use Pin 7 on the G540 as the gnd connection? Another thought. Since I am using a star type gnd where everything is grounded to earth, can I connect the C15 gnd pin to anyplace convenient on the case? The C15 also requires a 5V power supply. On the existing BOB these were plentiful, but does the G540 have a 5v connection? Or will a wall wart be required? The power supply is a 48V 7.3A model.

Thanks
Dave