From Ueno Station take the Park Exit which leads directly into Ueno Park. Walk right, passing the The National Museum of Western Art, and the National Museum of Nature and Science is the next building.
From Shinjuku Station to Ueno Station:
Take the Yamanote Line direct to Ueno Station (26 minutes, ¥200)
From Tokyo Station to Ueno Station:
Take the Yamanote Line direct to Ueno Station (9 minutes, ¥160)