It depends on the computer, but many of them can be set up in the bios so that the usb ports are only powered on when the pc is powered on.   All you do is cut the end off a usb cable, and connect a urc voltage sensor to the 2 wires that show 5 volts when the pc is on.  (usually the red and black wires on the usb cables i've cut open)  

As far as the power button connected to the relay is concerned, it is actually pretty simple, but will vary by PC.  you need to find the 2 pin connection on the mother board that leads to the momentary power button and tap into those 2 wires and connect them to the common and normally open connections of one of the relay outputs on the msc400.

you program an if/else statement to close the relay contact for about a half second if the voltage sensor doesnt detect voltage.  

you could also skip wiring the usb to a voltage sensor and configure the power button in the power settings inside windows to do nothing when pushed if the pc is on.  this way even if the pc is on, and you close the relay contact, the pc won't do anything.  but it will still turn the pc on if it was turned off.