Well, go to the mountains and find the solitary cedar tree. (to find it, investigate all of the tree you find in the mountains an investigete them or press the x button in front of it. If you found that tree, it will read "solitary cedar tree". then chop it with your axe. The tree will talk it will ask if you will chop it or not. Dont chop it! if you did not chop it, it will give you a power berry.
ps:can you chop it and tell me what happens? Dont worry, just load the game after that!