If you still have older version of Photoshop installed on your computer, or at least still have a folder for the older version in the program files, go there and find the folder for the Nik plug-ins. It will be in the Plug-Ins folder. Just copy that folder to the same plug-ins folder in Photoshop CC 2015. That's what I had to do.
You can either rerun the NIK install program, or just copy the plug-ins from the old folder to the new one.
As long as we are talking about Photoshop and not Lightroom....
You can even just create a shortcut of the filters in their current location, and copy the shortcut to the PhotoShop plugins folder.
That's what I do.
Thanks, ManiacJoe. I had to rerun the NIK install program to get the plug in