@rocksteadyfreddie facebook closed the loophole this extension was using to get the old design back., so it no longer works.
You should still be able to switch back if you are a Page or Group admin, just login to your page or group, and there should be a button at the top left letting you switch back to the old design for 48 hours. If you are not an admin, it should be possible to create a "shell" page or group just for the option, in theory.