That sounds like the power steering resevoir. Should look about the size of a small coffee can. Unscrew the knob and lift off the top. There is a line stamped in the body, about 1/2 inch below the rim, that's the fill line, the filter should be under the plate with holes in it at the bottom. Any quality power steering fluid will work.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.