Here's an English-language map from the Asia Center's own website--
http://www.asiacenter.or.jp/eng/access/
The same page contains suggested routes to the Center.
I've stayed there a couple of times, although not since 1991. The rooms are like small college dormitory rooms, and the cafeteria is only adequate (better for breakfast than other meals), but they're used to foreigners, and you're within walking distance of three major subway lines.