Take bus no.936. The no.6 tour bus probably is suspended these days. You can take bus no.936 and get off at Huairou Mingzhu Square (Huairou International Conference Center), and take a local mini-bus or hire a taxi to Mutianyu Great Wall.

It's a problem since you don't know mandarin. I suggest you write some simple Chinese words such as Mutianyu Great Wall on a piece of papaer and show it to the driver. Besides, remember to bargain a good price, or you can share a car with other travellers.