Hi, you 6 bus only runs on every Saturday, Sunday and public holidays, and it runs between April 15 and Oct.15. So it is not available now. You have to take bus 936 at Dongzhimen Station, but you should ask the bus driver whether his 936 bus goes to Mutianyu because 936 bus has many branch lines. The entrance fee of Mutianyu Great Wall is CNY 40. Yes, 936 bus accepts IC card.