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First, you may want to be aware that this transmission requires a special fluid; use of a generic transmission fluid will result in internal transmission damage.
Transmission fluid is added upwards, through the drain/ fill plug located in the bottom of the transmission pan. A special fill tool and adapter fitting are required to do this.
I realize this is not a very user friendly way of filling the transmission, but several other manufacturers have similiar fill procedures including Ford, Isuzu, and Honda these days. According to mercedes this transmission fluid is good for the life of the vehicle and therefore should never need to be changed; as a result they seem to not have provided any method of filling the transmission without the special equipment found at the dealer.