Sorry for the tethering issues.
All i can say as if now is that Nikon, Apple and Adobe. all three are working on it and trying to get it fixed as soon as possible, we will keep you updated.
As if now if you really want to use Tethering the only possible solution is to revert to Yosemite.
Hope this helps.
Talk to Nikon - the problem is with their tethering code.
See this topic in the official Adobe feedback forum: Lightroom: Can't use tethering with Nikon and Leica after installing MacOS 10.11 El Capitan. If you follow that topic, you'll be notified when the situation changes. (This forum is primarily a user-to-user forum and not every thread here will necessarily be updated when the status of an issue changes.)