If you get it working, consider editing the StackExchange post with the correct details for VS 2015 (the "community wiki" designation of that answer means that you are welcome to do this). Other people who use VS 2015 will appreciate it. I do not have this software myself so I cannot test, I can only guess at the correct setup.

I think there is very little risk to trying this out, just make sure to create a backup of the file you modify. You can run a text editor as administrator, then use it to open and edit the file.

There are good reasons why WRI discourages people from modifying installation files: doing so can break things, sometimes in very subtle ways, and people might not even realize that the problems they see could be because they modified something several months ago and then forgotten about it. Then this creates an additional support burden for WRI. So they certainly don't want such modifications to become commonplace in the community.

However, in this specific case, I wouldn't worry. Just make a backup of the file and do keep in mind that you made this particular modification.

Here's how to get Mathematica 10.2 to recognize MS Visual Studio 2015's C compiler (Windows 8.1).

• In the Mathematica installation folders, go to SystemFiles / Components / CCompilerDriver

• Make a backup copy of VisualStudioCompiler.m (for any disaster recovery)

• Open VisualStudioCompiler.m

• Scroll to $VisualStudioVersions and add "2015" to the list

• Scroll to installPath["2013"] and after it, include this new line

                installPath["2015"] := installPathFromEnvironment["VS140COMNTOOLS"]

These hiccups bit me:

• Although I could edit VisualStudioCompiler.m, it wouldn't save to the above folder

• Instead, save it elsewhere and then copy it into the above folder

• It's not enough to simply install VS2015 -- once installed, within it you must yet install the C++ compiler

• Once the C++ compiler is installed, open Mathematica and run:

        Needs@"CCompilerDriver`"
        CCompilers@Full

• You should see Visual Studio in the output list -- you're good to go

A big "Thank You" to those who led me to this solution.

Bruce

20150818