some reading around suggests your pc's power supply does not come equipped with 6 pin pcie dongle. there's also a suggestion that dell psu's are not the same size as non-dell psu's...(which if true is an absolute disgrace )
i've seen adapters online that will convert two 4 pin dongles to a 6 pin pcie....... but you don't appear to have two spare.
i'd say you're in need of a new psu and that you need to measure your current psu to be sure that your new one will fit in your case - if not then maybe dell can supply you with one that does.