Unscrew the black knob and remove the lid, then start the engine. There is a whitish plastic level indicator in there, the owners manual will explain how to determine if the level is correct.
I recommend the MB power steering fluid, not ATF. ATF is recommended on older MB's with power steering, (older than a 190).
If a dealer isn't convenient, go to a PepBoys or Autozone (or even a discount store) and get a container of what they call "power steering fluid", not ATF.