power supply not working

i recently built a computer for a friend...he bought the parts...i put it all together...and no i'm not a noob i have built all my machines...anyway he got a MSI P35 Neo2-FR...the case and power supply are a coolermaster...the supply is 380 or 400 watts...but it does NOT power the board...i have plugged in a small 350 watt el cheapo power supply and it works fine...he got another NEW power supply 420 watts...same issue....any clue as to what might be the problem?...bye the way...the motherboard has 2 extra hookups for power...1 looks like the kind on a hard drive or cd rom..the 4 pin.....the other is a 8 pin connector...the el cheapo supply has a 4 pin that fits into the 8 pin slot...the coolermaster has a 6 pin that fits in the 8 pin slot...odd that cheapo powers the board...thanx for reading..please help

Wait a minute...you plugged a six-pin power connector into the 8 pin slot?

OOPS...that was a PCI-E video power cable, not a CPU power cable. You may have fried something. There may be cap over 4 pins of the 8-pin connector; or if not capped, there will be instructions in the manual as to which 4 pins should be used if you only have a 4-pin CPU power connector (will be 4, 8, or 4+4 from the PSU, NOT the 4-pin part of the 20+4 pin main power connector).

Only four of those pins are needed. If you look at the picture provided by auscanzukus, you'll note that four of them are capped; that is for PSUs that have only a 4-pin CPU power cable.

Yup, 4 of 8 cpu pins are needed. Whichever is available in the psu. If none, you can pick up an adaptor for molex to 4/8-pin cpu power.