Hi there, To check transmission oil level you would first need to buy the dipstick or "measuring tool" as Mercedes refer to it as it doesn't come factory fitted.
You would first need to get the vehicle on a level surface and carry out all safety precautions before starting vehicle. You then insert the dipstick fully in before removing and observing the reading on the scale. The first reading should be taken from the cold temperature marking.
You then start and leave car to run for some time or maybe drive around to bring the oil to a temperature of about 80 degrees (check marker to confirm temp) and then measure the oil the same way as before except this time you will be reading the oil level on the hot temperature marking on the dipstick scale. It is also recommended that after this stage you go through the gears on the shifter with car stationary before taking another reading, hope this helps.
Kevin