The National Center of Sciences is a building located almost in the center of Tokyo. It houses several Japanese governmental academic organizations such as the National Institute of Informatics. You can go from Tokyo Station to the Center by taxi in about 10 minutes. It is called Gakujutsu Sogo Senta in Japanese, and a famous building, Josui Kaikan, is neighboring.
There are two subway stations near the Center. One is Jimbocho Station (S06/I10/Z07), whose A8 exit, located next to Shogakukan, Inc., is the closest to the Center. The other is Takebashi Station (T08), whose 1b exit, situated next to Mainichi Newspapers, is the nearest. See also information presented by subway companies Tokyo Metro and Toei Transportation. They also provide instructions for using subways.
If you want to go from Narita Airport to the Center, a simple way is to use Narita Express to get to Tokyo Station and then to take a taxi to the Center, which will cost you approximately 4,000 yen in total. An inexpensive but somewhat complicated way is to use Keisei Limited Express to get to Keisei-yawata Station and then to take Toei Shinjuku Line from Motoyawata Station to Jimbocho Station, which will cost you 1,150 yen in total. You will need at least one and half an hour in both cases.
Address: National Center of Sciences, 2-1-2 Hitotsubashi, Chiyoda-ku, Tokyo 101-8430, JAPAN