To adjust requires a 3mm allen wrench. Turning the adjustment clockwise richens, ccw is leaner. To know what you are doing there are two methods.
One involves monitoring the integrator on the #3 pin of the diagnostic socket on the left fender. This will require a faily good multimeter with a duty-cycle range.
The second and more informative way is to monitor EHA current with a cheap multimeter (one with a 2amp scale).
Both methods are described in the DIY article "evaluating engine controls". The value you are going for on an early 190 is 8ma with an oscillating reading (plus a ma or two and minus a ma or two). Remember that a dead (or disconnected - to verify your system try disconnecting the O2 sensor and you should get 8ma EHA current or 50% duty cycle on the integrator) O2 sensor will give a fixed reading of 8ma when warm (so it can't be adjusted by either method - both methods monitor the closed loop lambda cycle).
Bosch Master, ASE Master, L1
33 years MB technician