I'll make a bet here...
That power adaptor goes to the same molex (power thingy) that goes from your PSU to you CD/DVD ROM or even to you HDD if it is SATA with both connectors. Also, the Y adaptor has 2 of those molex'es.
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That's the molex i'm talking about (kind off, lol) that goes to the CD/DVD ROM. The adaptor should have 2 molex with that form: female and male. Plug the correct one to the one that goes into the CD/DVD ROM, and the other one to the power cable. The last molex (the 6 Pin or 2x3 Pin) goes to the card like you already know.
Hope it works.