Just installed this plugin, activated, and went to Appearance> Theme Test Drive. I selected my theme from the drop down menu and clicked "Enable Theme Drive". When the page refreshed, the "Access level" box had a 0 put in there. It's supposed to be a 10 so I changed it to 10 to indicate "only admins should see this theme". It does change the theme, but it makes it visible to all users not just admins.
Any ideas on how to get it working?