The girlfriend and I spent an hour trying to find the power steering fluid reservior on a Mercedez-Benz 190E (a.k.a. W201). Even the internet didn't know. We found it again, in hopes that the next guy finds it easy.
This is the power steering pump in a Mercedes-Benz 190E (a.k.a W201). The top of the power steering pump is the power steering reservoir.
Simply unscrew the black nut on top, and remove the lid. You should extract the old fluid using something like a turkey baster (which, of course, you should never use on food again). Then fill it mostly to the top (owner's manual says 0.6L).