After upgrading to Joomla 3.6 I had a nasty surprise - in the Joomla Installer the tabs - Install from file, Install from folder etc.., all but the Install from Web - where all gone. Of course, nothing found on the web: this happens when you are an early adopter. But I discovered the "fix" accidentally, on my own.
Trying different things, I cleared all the Joomla caches and my browser caches, and tried the Discover for new extensions. For my surprise, among the "discoveries" was the Installer plugin too.
Later the solution was posted in the docs.joomla.org too. I don't want to take any credits for it ;) but maybe publishing it there means one more chance for you to find it!