I'm sure you're done by now, but for future reference of anyone. You have the 4 black screws on the side with all the ports, take those out. Then the mobo is bolted onto it's own tray, it kind of slides out a bit, when it stops, lift it out and then voila, you have the mobo seperated from the case. To get it started you might have to slide your screw driver into the gap you made from the screws gone and leverage it a little.
*Edit - if it is the small form factor.