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

Thanks, Szabolcs. You're right -- "VS140COMNTOOLS" is correct, per the environment variable that I've just learned to access (Control Panel / System / System / AdvSystemSettings / AdvancedTab / EnvironmentVariables then scroll down in the box).

Now I need to unprotect VisualStudioCompiler.m so that I can edit it. Years ago, Wolfram Tech Support said "Check with us first before editing *.m files" so I'll wait 'til Monday to do so. I'll then post to Stack Exchange, as you requested (yes, that was indeed the thread -- I now see that you had participated in it).

Looks like I need to wait 'til Monday. No matter what text editor I use, the modified *.m file just won't save under Windows 8.1.

Wolfram Research is too clever to outwit.