Hi, guys.
Thank you for your interest in the extension. If you are trying to import an encrypted TiddlyWiki file, you need to put your password in before clicking "Import Current Page".
If you are still struggling, please post questions with as many details about errors you are getting as possible in the Tester's group that you joined.
As for the other questions:
The files are stored in the browser's local storage.
At the moment, the software is closed source.
At the moment, it's entirely free but there might be premium features later on.