Mike-on-Tour

Obviously it's enabled in customization, but when I go to extensions it's not available to modify. I've disabled it, deleted the data, deleted the folder from /ext and reinstalled it again. Same results.

Works GREAT on my test machine and I love it!

Have you read the README.md file? That's basically the manual for that extension.
If you can't see it on the extensions tab then you have to set the permissions for your respective admin role or admin group (Usermap works with the roles as intended by the creators of phpBB).