The left window shows the catalogues and disks catalogued, and the directory structure in them, if expanded. The right window shows the files in the directory that is highlighted on the left.
Things to note
Any file can be run directly out of the catalogue. Just double click the file you want to run, and the program will prompt you to insert the correct disk into the drive that you catalogued it from. When you press Ok it will attempt to run the file.
You can right-click on a catalogue or disk icon in the left pane, or any icon in the right pane for a menu of what you can do with that object. The menus are the same as the menu bar's "Catalogue", "Disk" and "File" menus.