The PS-2015.5 installs to a completely new folder structure in the Windows 'Programs' folder, without the plugins.
The install notice- told you this may happen.
It is easy to restart the NIK plugins-
1. Open the Plugins folder of the previous PS install- C:\Program Files\Adobe\Adobe Photoshop CC 2015\Plug-ins
2. Copy the 'Google' folder
3. Open the Plugins folder of 2015.5 - C:\Program Files\Adobe\Adobe Photoshop CC 2015.5\Plug-ins
4. Paste the 'Google folder.
5. Restart Photoshop.
I do not have an answer, but have experienced a similar problem some time ago. The windows registry still points to the 'old' folder address that no longer has the current PS version.
Read the Adobe Help page at- "Edit In Photoshop" command missing
(My fix involved un-installing ALL Adobe programs, running a registry cleaner, re-installing the CC app and Ps Lr Br.)
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Reinstall the NIK plugins from the Google site. You need to download the file then run it to install it.
The newest version of PS CC installs a complete new version in a completely new folder and for some reason it is not carrying over plugins.
Actually, the Nik folder is still in the previous Photoshop folder. All you need to do is find the folder and copy it to the new folder in the same location. Just take a look in Program Files.