The Jeweled Silver Monkey is the second treasure that can be found in Uncharted: Drake's Fortune.
The Monkey is found in Chapter 2 - The Search for El Dorado. The treasure is in the are where you have to push a large rock to break the wooden ground in order to enter the temple ruins. Make your way to the first stone that slips as you walk forward on it. Jump forward to the first pillar, turn left, and jump to the next pillar. The treasure is around the base of the tree. If you stand where you had to push the rock down and face the three pillars in the middle of the courtyard, the treasure is on the right pillar.