You'll see a family (mother and son) who can't find the father in Macalania Woods. Find the father in the "Spring" area, where you saw the Tidus and Yuna love scene. go back to the family, but now the boy is missing. Go up the extra trail (that passes over the forest and avoids encounters), and you'll find the boy up in front of that big glowing ball. Talk to him, he'll run back to his parents, and then press X in front of the ball to power up the Coudy Mirror to the Celestial Mirror. Of course, you'll need to have the Cloudy Mirror first, which you get from winning the Chocobo race at Remiem Temple.