From the Hachiko exit of Shibuya station, walk down Centre-Gai (the famous walking street next to the big Tsutaya and Starbucks) until you reach the end, and turn right. Then left at the T-intesection. At the corner with Tokyu Hands, turn right, and then left straight away up a little walking street next to Game/Chelsea Hotel/Milkyway. If you look up, you'll see a little sign indicating Organ Bar on the right hand side. It's on the 3rd floor.