From Ueno Station take the Park Exit which leads directly into Ueno Park. Once inside the park, the The National Museum of Western Art is directly to the right.
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)