I simply attached the ends of the coin door microswitch to a keyboard encoder (which gets it's power from the computer). This way, when you put a coin in the slot then the microswitch closed and causes the keyboard encoder to generate the keypress it which it is configured to generate.
If you would like to light up the little lightbulbs used for the coin door, then I would suggest to use the PC power supply. You plug the PC power supply into the wall and then attach the wires from the lightbulbs to one of the 5V lines coming from the power supply.