hmmn, from the sounds of it, you weren't using hill descent control properly.
it is designed, as per the title, to allow you to come DOWN a hill safely at a controlled speed, and will have no effect going UP a hill, particularly as it switches off when either the brakes or throttle are used.
to use it correctly on a failed hill climb as you describe, you should have switched HDC on and selected reverse. in that way, the uncontrollable slide you describe should not have nearly been so bad and you should have been able to safely steer to the bottom.
also, it only works in 1st or reverse. by putting the gearbox into park, you've just disabled it again and brought the vehicle to a complete standstill that way.
RTFM.
