You're going about it all wrong.
You need to remove the pan under the engine, then the other pan under the radiator and foglight (the bumber "apron").
Then you'll be able to loosen the radiator drain plug, but you also need to loosen the block drain plug, which is on the right side of the engine near the motor mount, to drain the block.
After it drains out, then you can start adding coolant and water in a 50:50 mix. You shouldn't be removing the thermostat or anything like that to do this job. You should also use the MB coolant.