How to move the stone?

I have a builder with nearly full experience, but when I put him over the stone left of the waterfall, he says we need to do more research?? HELP!

Perhaps you are jumping a bit ahead of yourself and need to do something else first. This isn't a hint but that is all that I can think of. Have you looked at the list of tasks at the bottom of the walkthrough page to see if you have done everything listed before it? I'm sorry that is not more help but that is all I can think of.