If it's a standard AT psu (then one of the cables going to the motherboard will have the color combination orange PwrGd, red +5V, yellow +12V, blue -12V, black Gnd, black Gnd) just wire up the PowerGood with any of the +12V whires. I've done that like on 4 AT PSU's by now and so far everyone have worked like a charm. If you can't find an orange line then it doesn't follow the AT standard but you can just open the PSU and look at it's board cause they usually mark it on there aswell (PG, PwG, PwrGd or simular). :-D
