they fit under the tip of the injectors, and they are NOT reusable. they must be replaced each time, or you will loose compression, and soot will damage the injector bodies.
if you click the mercedes link at the top of the page, and select your car, then choose fuel and intake, then choose fuel injectors, then scroll down a bit, you will see them.
note: the pic above is the side that faces the pistons, not the injectors. the side with the ring on it faces the injectors.