Downloading can be surprisingly difficult. The easiest way is to use the GitHub Radiance mirror. Since GitHub is a Microsoft service, Visual Studio provides excellent GitHub support. Downloading from the CVS original is also possible using CVS under Cygwin or Tortoise CVS.
Once you have downloaded the code, run the “cmake-gui” command, navigate to the “ray” directory, and run “configure” and “generate”. I have found it simplest to generate Visual Studio solution and project files. (You can also use Visual Studio’s own cmake support, but this requires one to know a lot more about cmake and VS than I do.)
Start Visual Studio and open the solution file in the “ray” directory. You can then build various versions of Radiance.
Happy hacking!