This is not a solution, but if you want to try here is how I did it. Remember, it doesn't fix anything.
After installing the Revit 2015 SDK, go to the Autodesk.AddInManager.addin file and open with Notepad.exe. then change the lines with the following location:
<Assembly>C:\Program Files (x86)\Autodesk\Add-In Manager for Autodesk Revit 2014\AddInManager.dll</Assembly>
to the following :
<Assembly>C:\Program Files (x86)\Autodesk\Add-In Manager for Autodesk Revit 2015\AddInManager.dll</Assembly>
This should allow the addin to run with 2015. But the 2015 SDK you are pointing to is programed wrong so it will seem to run correctly. But it puts the add-in into Revit 2014. They just shiped the same program and slapped 2015 on it.
SOLUTION??----->>>:
The best thing I came up with was to go ahead and run the add-in in 2014, then copy the files from this folder:
C:\ProgramData\Autodesk\Revit\Addins\2014
to this one:
C:\ProgramData\Autodesk\Revit\Addins\2015
Although "ghetto", it does seem to work for us.