I’m not sure why you are getting “Your file couldn’t be accessed” error. Judging from your previous description you were encountering a different issue, and then maybe moved the extension folder or renamed something in its path.
The original problem, it looks like, is due to you trying to download something from the new Memrise site, which is not what the extension was designed to do in the first place. The old courses it supports were moved by Memrise to a different domain since then, and now can be found here:
https://community-courses.memrise.com/community/courses