To compile SCSS in Visual studio, install the CompileSass Visual Studio Extension.
Unfortunately Web Essentials isn't going to support compiling Sass anymore.
I thought this was pretty sad because it was absolutely the simplest way to compile Sass from Visual Studio. I created a new Visual Studio extension that behaves the same way. You just install the extension and it will automatically compile .scss files to compressed .min.css files when you save.
Check out CompileSass.
