Checking AT Fluid Level
There are a few things to note here:
Everything inside the transmission must be as clean as can be, a single speck of dust can cause issues.
ATF reacts with air in order to purge the inside of the transmission. When you check the fluid level, you introduce air to the system.
The transmission must be warm (20-30min of driving), in P or N, and the engine idling.
There is a red cap to a dipstick behind the air filter, close to the firewall, near the back left side of the valve cover.
There is a lock on the underside of the cap that must be unlocked to remove dipstick.
Mercedes recommends using a piece of leather to wipe the dipstick, to eliminate possible fibers.
Fluid level should be at the higher of the two marks at operating temperature
Star Wiki has a good section on the 722.3, including this part on filling and level checking.