Monkey Mountain is not an official name, it is just a nickname, hence not on a map.
to access the area, take the old Tai Po road, marked as tai po road - pipers hill on the ypmap.com towards Shatin.
From kowloon direction go to tai po road (very long raodZ) you take left off tai po road towards the kowloon reservoir road bridge into Kam Shan country park is on the left, with Lion rock country park on the right.
Beware! Those monkeys are rascals! I've seen them attacking and scaring people more than once. Once a man was being scratched on the back with a monkey's claws and you can see the blood and a torn T-short. Another time an old lady alone got off the bus, before she could ask us for directions to the monkey mountain, the food in her Wellcome bag was snatched by one monkey and his whole family were enjoying it, while the poor old lady stood shivering. Another time a sneaky monkey wanted to steal our food while we were fishing.
Avoid taking Wellcome and Parkn'shop bags in all case. They're targets of attack.