You are correct,,there is a knock-out plug at the base of the dishwasher drain line input port to keep the port sealed when there is not a dishwasher.
You can use an old screwdriver and a hammer to knock the plug out. Insert the screwdriver in the dishwasher drain port tube from the outside of the disposal until the tip is in contact with the plug, then tap the screwdriver with a hammer and the plug will pop off.
You will then need to remove the plug from the inside of the disposal body before operating the disposal.
