I have a 560SL roadster--same engine I belive. There is a separate drain on each side of the engine--they are reached from the bottom of the car (flat on your back looking up) about midways back on the block. They require a 10mm hex drive socket (pretty sure that was the size). They are actually plugs (about size of 50 cent piece) with the female hex opening. Mine came out fairly easy, but have heard others say they can be stubborn.
Word of warning--clearance is not real good on the drivers side and you will need a fairly long extension to get up in there. Be extra careful and make sure you get the hex drive seated in the opening, or you can easily round it. When the plug is completely removed, get ready for a bath--it's a big opening and the water floods out. Good luck. John Downey