Have you read the accompanying readme.html? If you have, please tell me where did I lost you, so I can improve it.
Basically, to get the JSON file, you have to set a savegame as the "input", then set a filename as the output. The app will detect this ad disable the "import" button, so you can only either press "Export", or "Exit". If nothing goes wrong, you will have the JSON file as indicated in your "output" field.
To import back, just reverse the process, putting the JSON file in the "input", and a savegame in the output. Mind you, the app will reject if the output is the same file used to create the JSON file.
Yeah, currently the program is complicated, needing the JSON intermediary. A full blown GUI is in the plan though, one that doesn't need JSON import/export, but I planned to write that as soon as a full savegame reader/writer is written (which is in the works now) and tested.