You should go to Dongzhimen first. Bus 936 is in Dongzhimen Coach Station, not in the bus stop outside subway station. You can also take bus 916 at the bus stop. There is not direct 936 bus to Mutianyu now. It stops in November. So, take either of the bus to Huairou Bei Dajie and then hire a van or taxi to Mutianyu. This is the easier and cheaper way. Your hotel is close to Dengshikou Subway Station of Line 5. You can take subway to Dongzhimen via transfering at Yonghegong.