First you must go to the Pokemon center in island 1.Talk to the man standing in front of a machine (Nicichi)he will tell you that he needs a stone.After that go to light mountain, there you will see two team rockets,beat them in battle,then they will disappear after you defeat them,there you will see a cave go inside and get the ruby stone.Go back to the pokemon center and give the stone to Nicichi then he will give you the Pass which enables you to go to the said islands.