Solder the LED to the base of the 5v and ground pins, take care to remember which side is anode (+) and cathode (-) so that you solder it right and then after bending the tabs on the pins back outward carefully cram it all back together. I used some silicone around the molex to cover the bare LED leads. When the molex ends were all outfit with their own LED I used some chrome loom to cover all the wiring.
There are other ways to accomplish this mod... at least insofar as the voltage of the LED is concerned, if a 12v LED is used it can be hooked to that line (yellow wire) and a ground (black wire) or you can add a resistor to the anode side to make the LED run at 5v (red wire) which keeps everything cleanly in the middle of the molex. If you need to find the resistor value for your LED there are many LED calculators on the 'net.
I did this mod to my test PSU and it does look sweet, if I were to do it for my own case I would only put LED's into the molex ends that hooked into my drives since they are the only ones that would show through the side window. This mod is cheap and easy, if you cannot hide ugly plug ends then you may as well turn them into something worth seeing... sounds like some of the women I've known, LOL.
Credits: To ArtTROY
Happy Modding