For users who cannot see the button even after following the previous steps, try reinstalling and installing for "All users." Sigh. I wish I had time to work through the nuances of Visual Studio setup projects, but that is outside my realm of expertise.
This experience has given me directions to test these addins better in the future. Setup testing is going to be the most time intensive part of this testing, though, so I won't have as much time to develop and test these things.
In other news, someone beat me to the Matlab addin I was thinking about. Check out http://www.onenotepowertoys.com - pretty slick. This will make likfe a bit easier for college students trying to keep math notes together in one location.