You may have missed this:
Move that file to any shared folder you can both access.
The purpose of moving the extension somewhere
other than its correct and final location is to give the OS an opportunity to do that job.
Many extensions can be loaded in the correct location by simply double-clicking its icon. The OS then '
sees' that it is something that needs to be put in a certain System folder and it will do that for you. This sometimes allows you to start using that extension immediately. At other times, it requires you to go through a Start Up process which allows the OS to note which extensions, daemons, scripts, etc. should be active.
You may have better luck following this 'automatic' process rather than manually moving things where we think they should be. Moving is but one part of the installation process. Updating the OS (through a Start Up process) is another part.
When you install some software (especially as the Administrator), you often get a dialog asking if you want to install only for your use or for "All users". I usually use the latter option as I may want to boot up as a different user to test for problems in my normal user account.
You are almost there! Hope you get things back in good order before I return from a vacation in January!