Um...

Who told you about the cube? Did you just find it?

If nobody told you to get the cube, then you don't need it yet. It'll just be handy when some robot says, "You need the cube for passage to the Sky Castle."

And if you want more direct answers, read a walkthrough. I thought I was only here to give you small hints.
If I were you, I'd be hunting squid right now. But you may have already done that. How about, I dunno... Reading about my location.