you don't have to worry about the 5 volt switch signal for the ATX power supply.
There isn't anything to worry about. You just connect the green wire to ground on the ATX connector and the supply powers up.
If the PSU has a separate mains switch, you can make the connection permanent and use the mains switch. If it is "always on" when it's plugged in, then just connect a switch between green and ground.
