You should be able to download the donation key file, and place it in the same folder as the exe and pck file. When you launch the game with the exe, you should have it.
I think the above person installed the game through the itch.io app, rather than downloading it through the website. Installing it through the app does nothing, and gives an empty folder; downloading it functions normally.
Also, the key can be obtained without paying for it presently, due to the ongoing dispute with Stripe. It might be worth considering adding an alternative method of obtaining it.