Power connectors are keyed to only fit one way. Do not force anything.
Look carefully at the shapes of the 8 pins/sockets. Some will be partially rounded to insure that they fit only one way.
There is a latch to keep the lead in, it locks on to a small ledge on the card. Make certain it is on the proper side.
Since a 8 pin pci-e connector may be unusual, some GTX580 cards will come with an adapter .
Because your psu is modular, make certain that the power leads have the proper end plugged into the psu.
If all else fails, read the manual.