I think you might be doing something wrong then? Maybe you have the wrong file? The Kindle For PC app should always give you azw files, not azw8 (even if it does use the newer format they're all just named azw).

It should show up in the epubor app on the Kindle tab after you install the kindle app and used it to download your books. No need to drag and drop from the file system it's all right there in the app. It finds Kindle, Kobo, etc and lists them.

EDIT - Make sure you leave the Kindle app running. I think it needs to be able to read the keys from memory or something.