It's a 10 pin IDE connector. Any decent electronic supply store should carry it. You'll need some 10 conductor ribbon cable to go with it. According to the manual, here's the pinout:
1) +5v
2) USB D-
3) USB D+
4) GND
5) GND
6) MOBO POWER
7) MOBO POWER
8) AMP THUMP
9) GND
10) NO PIN
1-4 are for programming the supply thru a USB connection. Not much detail on that, tho. Your power switch hooks to pins 6-7 and your amp relay goes to pins 8-9
It's all spelled out in here:
http://resources.mini-box.com/online...ATX-manual.pdf