If your psu does not have a 6 pin pcie cable, consider the possibility that it is an old design that may not be able to deliver enough +12v amps to run anything that needs a 6 pin connector.
Exactly what psu do you have, and what do you intend to connect it ot?
If your psu is of good quality, there should be 2-4 molex adapters that can be combined to output to a 6 pin pcie lead.