2003 ML 500 Steering Problem

Hello, I've recently started getting a problem with my power steering. My car is making a very annoying whining noise when turning. I purchased the correct power steering fluid, but am not sure where to put it. I noticed that there is an unmarked cap next to the oil cap and believe that this is where it goes, especially because its empty. However, I don't want to do anything unless I'm positive. Please advise.

there is a black reservior on the front of the engine, driver side. top off the fluid and get it checked to see where the leak is coming from. common areas are a reservoir seal, and a power steering line