If you have your project details displayed, it's easy to load a file for edit.
Just double-click on the file and an appropriate edit window opens up.
Here, for example, you could double-click on STOCKIOA.CBL and edit this COBOL source program.
You can use the + and - boxes to open and close branches of the tree structure so that files you're interested in become visible and those you don't currently want to see are not displayed.