Check the 8-pin power connector running up to the top lefthand corner of the board, next to the CPU. You want to make sure it does NOT say PCI-E on the connector. Look for another 8-pin connector if it DOES say PCI-E. Make sure you're using the split EPS connector.
EDIT:
I'm talking about the cable at the top of the frame of this picture. It should be an 8-pin connector that splits into 2x 4-pin connectors, that comes directly out of the powersupply. It should NOT be one of the cables that can be disconnected.