If you installed the DB link 2016 version you should find the "Autodesk.RevitDBLink.addin" file in the following path:
C:\ProgramData\Autodesk\Revit\Addins\2016
If so try to copy it to the following path:
C:\ProgramData\Autodesk\Revit\Addins\2015
If you have the 2014 version then do the same (copy from C:\ProgramData\Autodesk\Revit\Addins\2014 to C:\ProgramData\Autodesk\Revit\Addins\2015)
See if either could be compatible with the 2015 version and give me your feed back.