dumb question, but you did remove the plastic sheath (4 screws) first before you tried to locate the oil drain plug, right?
Once doing so, you should be able to locate the drain plug by crawling underneath the car (in front of the drivers side front tire) and accessing the drain plug. The plug will be located somewhat perpindicular to the ground if I am not mistaken.
'89 420 SEL
'90 300 SEL
'68 Olds 88 Convertible
'84 300 SD (sold it)