To ventilate the cabinet properly remove both the exterior and interior barrel plugs from the vent connection that you are venting.
The lower vent connection should be used for heavier than air vapors and the higher vent connection on the back wall should be used for lighter than air vapors to obtain the best fume removal within the cabinet.
The cabinet vent connections are designed to accept 2" diameter threaded metal pipe material, which can be purchased locally. Once the connection on the back of the cabinet has been completed, remove both the internal and external plugs from the remaining vent connection to allow for proper airflow from the cabinet.