Sure enough you’re going to see the empty folders, since you’re clicking the ‘…’ button to select a folder where cdda game is located. It tells you to select a folder, it’s not going to show you any files. To find them and to find the mods.json, you should manually find the way to that file using the Windows Explorer or another file manager.
Are you telling us you don’t know how to use the Win Explorer? Because the path to the game is clearly written next to the ‘…’ button.