You must have first completed the Bombers challege as a Deku scrub(finding the Bombers and getting the password)and can change back and forth between Scrub and human. Go into the Bombers hideout in East Clock Town near the Mayors Office then get the Moons Tear,take of the Deku mask so you are human and walk up to the Bomber guard at the entrance to the hide out and talk to him he will congratulate you and you will be able to pass in and out of the hideout freely (with the password ofcourse)