The 750w psus pretty much come standard with 8-pin PCIe connectors, as well as the 8-pin or 4+4 pin cpu power connector. Do not get those confused, they do not fit in the wrong connection as they are keyed with square and 'D' shaped pins. If you are positive you have the right connector (should have Eps or Cpu etc, not PCIE) then the 8 or 4+4 pin will align with the connector in the top left hand corner of the motherboard. And yes they can be difficult to seat and take a very firm push.
There are some exceptions, such as my MSI mpower z77 board that have a secondary PCIe 6 pin connection on the board, just to provide more stable power to the pcie slots, but that is mainly for sli/crossfire support and not to be confused with the EPS...