Got the same problem, since the registerapp.dll is gone in the sdk.
Tried upper solution, for a background addin.

The keys in the registry were made, but the addin doesn’t start.
… and is not listed in the settings menu.

When passing the registerapp.xml to the registerapp.dll there were more information:

What about the publickeytoken, the culture, the version.
How to move the dll to the gac?

Mic