To engage the parking brake in a Mercedes SL 450, first ensure the vehicle is stationary and the gear selector is in "P" (Park) position. Locate the parking brake lever, which is typically located between the driver's seat and the center console. Pull the lever upward firmly and release the foot brake pedal. The parking brake indicator should illuminate on the instrument cluster, indicating that the parking brake is engaged. To disengage the parking brake, press the foot brake pedal and push the parking brake lever downward.