You will come to the dream scene during the main story of the game. You will be in a maze like area. You need to find two frogs, one in the top right, the other kind of in the center/right. They will be standing still. They will give you clues which are pretty easy to decipher. Their speech will have certain letters highlighted in a different color. Not sure if it is random or always the same, but mine were
XIII (13) and R, O, T.
You then must go to the very top, there will be a bigger open area with lots of square stones and another frog at the end. He will ask for a word and a number, both of which you should be able to figure out from the two clues.
I marked my answers in a spoiler tag for those who want to figure it out for themselves. Again, not sure if it is random or not so please post if you got the same as me and I will update my solution accordingly.