Nikon service center used to lie in the Photo Department of Xinhua News Agency, but it just moved to a far place in Haidian last month. You can go to the Nikon Center at Oriental Plaza (a big shopping mall in Wangfujing).
It is difficult to tell the exact address, the repair service is at Room 307, E1 Oriental Plaza. (Tel 010-85152268, according to Chinese photography online forum, I have not check it yet). They also have a exhibition place in the Oriental Plaza. BTW, they do not work on Sat and Sun.