That looks a lot older than mine, based on the pictures, but if you are using the 1500 or 2500 series tanks, the upper port is plugged, and the tanks (cogs) attach over the lower port. The clip mounts below the port. You have to change the rollers for different sizes of tank. The 1500 series uses rollers on the extension arms, and the 2500 series need the rollers mounted directly to the bar on the lift support. This is based on my red CPE2+ with Lift.
If you see the reels when you look inside the top of the tank with the lid on, then either the center column is missing, or the lid does not have the funnel inside that inserts into the center column.