If there is no switch then it probably has a proprietary connector on it somewhere.
You will have to find out where that is, and how it works.
Try and find some documentation on it on the net somewhere.
If it does not have anything like that, I would imagine it has a switch somewhere on it, or it was expected to be unplugged, or used with a power strip.
Proprietary = bad.
zmoz has one that does not conform to standard AT spec, so teh wires are different color, Delk is usung ATX connectors wired wrong! so they fry your nice new motherboard if you do not know aout it and change it.
Since you already know its not standard, I would try to get it powered up, and then double check all the voltages before plugging anything into the board connector.
By the way, if its not powering on when you plug it in, some PSU's require a load on the 5 volt line in order for them to turn on.