To add power steering fluid, remove the black plastic nut and then the fluid reservoir's metal cover.
If the vehicle is exhibiting chronic PS fluid loss, inspect the pump supply hose coming from the fluid reservoir. M-B issued a Service Information bulletin regarding the M119 PS system; apparently incorrect length hose was used during assembly, which left the fluid delivery system prone to leaks. Repair involves removing the fluid reservoir and renewing the hose, reservoir base gasket and a couple of seal rings.
The PS pump on this engine is also prone to leaking over time. Factory reman pumps are available (but expensive).
The V-8 M119 uses a different water pump than the I-6 M103.