From Korakuen Station, take the Koraku Exit and turn right. Follow the train tracks for five minutes and you will see the entrance to the gardens on your left hand side.
From Shinjuku Station:
Take the Marunouchi Line direct to Korakuen Station (27 minutes, ¥200)From Tokyo Station:
Take the Marunouchi Line direct to Korakuen Station (9 minutes, ¥170)