IIRC, An ATX Power Supply can only be mounted one way. Looking how the screw holes align in the back will tell you which way to put it in. Also, they are all about the same size by the ATX spec so it will fit.
BTW, Looks like a nice case. I've got a few Antec ones as well although not that nice. I don't know much about that PS, but I just picked up a Corsair PS that is working fine so far.