SDK download
Why in heaven's name would you want that?
It only contains documentation and samples.
All the code you need to execute your add-in in any version of Revit is contained in the Revit executable installation itself, in the Revit API assemblies.
The old documentation is of very little use to you, except for informational purposes.
I cannot image why you would want it.
To compile for Revit 2015, all you need are the Revit 2015 .NET API assemblies.
As a minimum, you need RevitAPI.dll and RevitAPIUI.dll. They are referenced when compiling your add-in DLL.
To run and test an add-in for Revit 2015, youalso need to have Revit 2015 itself installed.
The SDK will not help you in the least with any of those two steps.
If you are absolutely sure that all the Revit 2015 API functionality used by your Revit 2015 add-in is also provided by the Revit 2016 or 2017 .NET API, you can also use those .NET API assembly DLL.
