its two little pins sticking up out of the mb , usually in the same block as all the other front panel switch and led pins .
It will be near the front of the case and at the bottom of the mb
Probably have LED or PWR LED written on the board in writing
If thats a bad answer , thats because it was a hard question to understand
LED are polarity sensitive, plug it in one way, if it doesnt work flip it around the next time you open your case, it wont hurt anything the LED just wont work.