So if the PC powers up with the power button, that means you have the power button connected to the headers on the motherboard, right? Will, if there's a power button connected there, that means the wire from teh Opus PSU is not. It needs to be wired to the power switch headers on your motherboard to send the signal to the motherboard to initiate bootup.
That's what #1 is saying. So open the system up, find where the power switch is connected to the motherboard, disconnect it, and wire the two-wire connector for the OPus and connect it where the power switch was.
I firmly believe that anyone attempting to put a Pc in their car needs to know this basic stuff about putting a PC together, as well as all teh functions and what wire goes to what and why. You obviously don't know that, so I suggest getting some education in basic PC hardware.