This is copied from one of my earlier posts....
I just did this for the first time myself. Couldn't find the block drain on my own from above, but a Mercedes tech showed me where it was while on the lift. You can not see it from above, but if you touch and follow the copper pipe coming out of the bottom of the air filter housing, it will lead you to it. If you MUST do it from above without a lift or jacks, disconnect the intake pipe into the air filter housing (no tools needed) to give your arm some room. Also, be VERY careful NOT to lean on the overflow hose coming out of the radiator. I broke the nipple off from leaning on it for leverage and am having a heck of a time fixing it.
TECH TIP... the MB techs use SHOUT (that's right... the stuff from the grocery store) in a 1/1 or 1/3 mixture for internally degreasing the engine before using a descaler or flush chemical. Mercedes recommends a 10% citric acid (quite weak.. ie white vineger is 5% ascetic acid). They usually don't bother with the Shout unless they have replaced a head gasket or something, but you may want to do it at least once. MB in their manual says it is critical before doing a chemical flush or descaling. They refer to it as "de-oiling" the cooling system. Also, it is important to remove the thermostat in order to do a proper flush and rinse, but you must put the seal back in without the thermostat while flushing or it will leak there.
Do multiple flushes between each stage including running the engine up to temperature with the heater on full blast for 5 minutes or so.
After this procedure your engine will smell sweet as your fresh laundry!!!
And finally, do yourself a favor and do an engine steam clean or degrease before you begin. It is really messy down there!
I'm sure this is more than you bargained for but I wanted to share my experience with you. Have fun!
Here's a thread on Shout for flushing...
SHOUT for coolent flush? MB recommended!