There are 3 drains-one at the radiator (likely near the lower rad. hose on your car; my experience with this motor is in my 500E), and two block drains. The block drains screw up into the block, and are aft of the motor mounts (they are Allen bolts, with non-reusable washers specific to this application; be sure to have them in hand before you begin). The service CD says to drain the coolant at these 3 points, then re-fill.
I suggest you consider doing this process twice, as a fair amount of coolant will remain in the system. I did a lengthy post on all of this in Hot Rods-check it out:
500E Coolant hoses