How to share files with an iPad app in El Capitan itunes?
It should still be there on the Apps panel, though it's more confusingly-located than I seem to remember it used to be. You have to scroll right down past the Apps section where you can add/remove/arrange apps, and you'll eventually get to a File Sharing section with a list of apps that support file sharing.



If you found your way there but the list is empty, then something else is afoot... if nothing else, you should see the iWork apps there, if you have those installed.