What is required to activate the "PS_ON" on the motherboard, to power on a computer? Looking to use a microprocessor/accessories to power on a pc
Are you trying to emulate a power button press to turn on a computer, or do you just want to turn on the power supply by itself?

For emulating a button press, I would recommend either a relay or an optoisolator.  I don't think there are any standards for how the power button on the motherboard is set up, so it would not be advisable to rely on, say, one of the pins being ground.  

For turning on the power supply, all you need to do is tie the PS_ON signal to ground.  This signal must be held low to keep the power supply on.  Since this is a well-defined standard, you can use a single NPN transistor to pull that pin low under the control of a microcontroller or similar.