I'm assuming that when I hit that power button, there needs to be circuit completed on the Mobo before it actually begins the start up process?
In the case of ATX powered systems, the power on button on your computers case is wired directly to your motherboard to two pins, which creates a momentary closed circuit, thus sending a signal to your controller chip. Once this is done, the controller chip sends a signal to your power supply (PSU) which powers on the PSUs power output. The switch in question is only a momentary switch, once it is released the circuit is now open, as it should be, because it has done its job of powering on the system.
If you have a manual that came with your motherboard, it should show the motherboard layout for connecting the various features found on the computers front panel. Such features are the harddrive status LED and the power LED. There is usually two wires wired to the power switch, however some have three or four wires so sleep mode can be activated through the same switch by depressing it in what is called soft mode.
You want to check this wire, your motherboard manual should show where it is located.
Also some systems have the ability of being powered by either a ATX power supply or a Baby-AT type, if yours has such a feature, you will find a jumper located on your motherboard that is either for ATX or Baby-AT type power supplies, if this jumper is missing or loose, you will not be able to power on using an ATX power supply. Please check your motherboard's manual carefully for this feature.