Dutch, the empty key to replace crystals in was in the granite house near Nemo. Just make sure that when you kill each robot that you take his key. You will need the crystals from them. There should have been a robot outside the granite house hiding behind the arch, one that could only be killed with the cannon, one near the rope bridge, one near the windmill and one near the lake. Each one had a crystal key and you needed to take them. You also need to take a couple of their thrusters.
I there still a monkey in your way at the back entrance to granite house? If you've still got a snake, use your snake on him or feed him.
If you have a screwdrive that you took from the granite house, pop the crystals out of the keys that you took from the defunct robots. Once you get Nemo's key, remove the spent crystals from his key and replace them with the glowing green ones that you have taken
Yesterday is but today's memory, tomorrow is today's dream.