The Golden Inca Cup is the fifth treasure that can be found in Uncharted: Drake's Fortune.
The Cup is found in Chapter 2 - The Search for El Dorado. You can find it in the room immediately after the dramatic collapsing of the wooden platform, immediately after the Golden Man cutscene. After the cutscene, turn around so that you can see one of the ceramic jars to the right hand side of the room. The treasure is behind this jar and on the body of a skeleton.