I have dove to the submarine and climbed on and stepped on the red light. I turned around and the robots were let go. Automatically I got away and am back in the Granite House . The walkthrough says to look at the Mysterious Key in the inventory but I haven't got it. Was I supposed to pick it up somewhere? Where do I find it?

Looking at a walkthrough, there is a mention of a key that you pick up, as follows:

Oh ! I missed that! and I read it twice! Sheesh! I have circled that on my walkthrough. In fact I had missed something earlier in the game and had to go back to a saved game. So I am going to go through and underline each item so when I play again I hopefully will pick up everything! Thanks Marian!

You are most welcome! Enjoy the rest of the game.