I have a Dell server mobo that I bought bare, and was in the same situation. I was never able to find the pinout though. I ended up resetting the CMOS, which allowed it to turn on as soon as power was ...

Use a jumper or conductive object, like the tip of a screwdriver, to create a momentary short across the power switch pins on the motherboard. That will start the system. The same should shut it off.