I wish I could be more specific unfortunately I didn't take notes when I started digging for things with "maya" in the name in the registry, and c: drive.
All I can really describe is after installing the extension for the first time, I kept looking for the send to unity feature. It wasn't in my file menu. So after looking in Help>about maya LT to verify I had the new extension correctly installed, I also checked to see if I could export a fbx file larger than 25k, and I could.
So then I tried uninstalling and reinstalling, still didnt work. But I noticed when I restarted maya lt, that it was remembering my project settings etc from the old install. So I knew then something was still floating around my hard drive. So uninstalled, searched for anything maya related, and deleted it. Then reinstalled, and it was there
!
It was just strange the help>about maya LT showed the extension was installed, I could export the new larger file for fbx, but my file menu didn't show the new "send to unity" feature.
P.S. I am not suggesting search and destroy missions like I did for other people, it can be dangerous! 