You should not be able to look down inside the magazine tube at all. It sounds like you are missing the magazine tube plug all together. The screw you see is the screw that holds the magazine tube plug to the stud that holds the magazine to the bottom of the barrel.
Simply go to www.e-gunparts.com
(Numrich Gun Parts) and go to the Marlin section and order a magazine tube plug. You already have the magazine tube stud and screw, so all you need is the plug.