Get off at Keio Line Hachimanyama Station (about 20 minutes from Shinjuku Station) and walk 2 minutes from the station.
(Rapid and regular trains stop at Hachimanyama Station.)

Exit the Hachimanyama Station ticket gate and go straight to the left.
When you reach the Koshu Kaido, turn left at the intersection and go straight for about 20m to the 6-story building.
Enter the left side entrance when viewed from the front, stair up to 5th floor.