The Plug "P5" is just another 4-Pin Molex power connector. It is used to power a secondary hard drive, and doesn't need to be connected to anything for the computer to work. If the computer isn't powering on, a bad power supply may be to blame. The PowerMac G3 Blue and White (as opposed to newer/older powermacs) uses a standard PC "ATX" power supply. You can buy these at just about any store that sells PC parts, or use an existing one if you happen to have one handy.