Though I haven't tried to remove the door sill plate out of my CE yet, I removed it from my E by VERY carefully pulling it away from the floor. It was attached to the carpet over-lapping a metal "rail" by pressure, similar to the way a plastic "binder" holds in paper in one of those cheap "book report covers".
Key thing to remember is to lift very carefully so you don't damage the plastic, which can get brittle with age.
85 300E euro-spec., twin-turbo AMG aero kit
89 300CE AMG aero kit