Right now there is no version of Lightroom 6/Lightroom CC that allows tethering Nikon cameras in El Capitan.
I have heard that they are working on a fix and expect it to appear in the next release of Lightroom.
It amazes me that companies as omnipresent as Apple and Adobe have allowed this to happen!
The problem is due to Nikon's software that allows tethering had a bug making it inoperable in El Capitan
>The problem is due to Nikon's software that allows tethering had a bug making it inoperable in El Capitan
Nikon was using a long deprecated API in their SDK and Apple finally removed it from El Capitan. They had long signaled they would but Nikon is not known for their up-to-date programming to say the least. Nikon only released an updated SDK for tethering their cameras that now works with El Capitan just before the holidays. Adobe has promised the next version of Lightroom will have tethering support built on this new Nikon SDK: Lightroom: Can't use tethering with Nikon and Leica after installing MacOS 10.11 El Capitan
This is the disadvantage of Adobe using vendor supplied SDKs to support tethering. A very few other software packages had no problem supporting Nikon tethering after the El Capitan update as they were coding their own camera interface. However there you might have to wait longer for a new camera to get supported.
I've been waiting for months for Adobe Lightroom to correct this issue with no luck. I just downloaded PhaseOne's Capture One Pro 9 and was up and running in 15 minutes with my Nikon D800.