Thank you for the inquiry and for using SearsPartsDirect.com. Sorry to hear about the issue releasing the jack, that is definitely an issue, especially if you need to use the car sometime soon.
The release valve is located on the power unit that you pumped the jack up with. You will need to use the other side of the handle that you used to jack the car up and fit it over the release and twist the handle counter-clockwise to release the pressure from the power unit. Be cautious when releasing the valve as they can release quickly and the car will drop abruptly. I like to take small twists at a time, and let it come down gradually.
Hopefully this will help get the car down and let me know if you have any further questions. Thanks for using SearsPartsDirect.com.