See the following:
Best ATF for MBs? Synethietics?
As to being low and smelling. You really need to addres the reason(s) as to why it's low (fluid loss rate?) and the smell (age?).
Read your manual very carefully regarding the amount to fill. Pay very careful attention to the use of the dipstick. Temperature is a big factor.
As a general rule, when I fill my trans after a fluid (pan and torque convertor) and filter change, I stop short 1/2 quart. Try to change it with the car level and the trans well warmed up (Larry's oil change advise applies here too).
Good luck.