Check the link to Wolgan's page below.
After MY2000, the ML transmission is what you might term "Shiftronic" meaning manual gear selection is EZ and fool-proof.
The OP '99. you could use manually with a bit more hassle - my suggestion would be to do a board search and re-set the transmission to learn mode, meaning let it learn frokm you how to be more conservative.
Mercedes-Benz Transmission Shifts