Ok, I might have overlooked this as obvious.
http://imgur.com/a/qrvwV
Imagine that's your computer, just use a paperclip or a piece of wire to connect those 2. Any black will do, and there's only one that's green. I'd suggest you do this with the PSU disconnected from the power cord (or if it has a switch, keep it off first), make sure the clip or wire went all the way in and it's touching the metal and then put the power back into the PSU. It will turn on as soon as you connect it, just be careful not to let it slip off as you could end up shorting other components.
As I said, I do NOT recommend doing this as there's too much risk involved, but it works.