Solution
It is not possible to open TestStand project files directly with TestStand. To display the contents of these files, you will need to open the Workspace pane (View >> Workspace) in the TestStand Sequence Editor and open a TestStand workspace file (.tsw) that contains references to the TestStand project files you want to explore, as illustrated below.
Alternatively, if you don't have a workspace associated with the project files, you need to open or create a new one in the Workspace pane and add the existing project files using the contextual menu, as shown in the following image.