Changing radiator how to top up the transmission fluid on my 1999 mercedes e430?

Does the car have a transmission dipstick? Look on the passenger side of the engine compartment towards the firewall to find the dipstick tube. If it has a dipstick there will be a square top on it with a locking mechanism. Run car to normal operating temerature, unlock the dipstick pull it out wipe it off push it back in and pull it back out then look to see if the fluid is between the marks. If not just add fluid through the tube.  If you don't have a dipstick there will be a smaller cap with  red plastic lock on it. You will have to break the red lock to get the cap off. There is a special tool from Mercedes to check the level on these non dipstick units. If you think it is low then add 1 quart and road test for proper operation and shifting.