The water from the pump comes in the filter in the center bottom of the barrel right under the air blower manifold through the 2 inch shower drain. The air blower manifold has legs built into it (4 way "T"s) to keep it and the grate off the bottom of the barrel.
Water goes out of the filter either to the pond via the top 3" line left side in picture or to waste via the lower 3" line, right side in picture. Look again at this picture, there is a plug that can be moved from waste line to pond line depending on where you want the water to go.
I posted pictures of one of these I cut apart a couple of weeks ago. It had been in service 6 months or so and the layers of sand and gravel were still inplace.