I just got back from Mutianyu. At the Dongzhimen bus station I was unable to find any 936 buses. No one knew of them not even the police information booth. If there is some trick to this I would love to know because I spent 45 minutes looking for a bus to Mutianyu with my bad Mandarin and failed. I ended up taking the 916 to Huiarou and then had to pay too much for a car to the wall.