Mickey is on the right track....
The transmission dipstick hole is protected by a cover, that is locked using a locking pin, the pin can be black or red color.. you have to destroy the locking pin first to remove the dipstick cover. After that, the stick is measured while the engine is at operating temperature (hot), and at cold temperature, so you have to measure twice because there are two measurements on the stick.. And only use Mercedes-Benz Transmission "Sythentic" Fluid, NOTHING else.. they are $25 canadian a quarts. your car uses 8.5 quarts) Reason why MB didn't include a stick? Because your transmission fluid will last you a lifetime, no need to change it! I changed mines at 90,000 kms because I love my transmision heheh, I sell the ML transmission test kit, comes with dipstick, 5 locking pins, and the complete instructions (as you can see from the picture) from the tech book.