C/C++ has never been supported in MonoDevelop on Windows, though reportedly some users were able to get it working with a mingw32 environment. Unfortunately the addin was accidentally included in some of the MonoDevelop 3.0.x releases for Windows.
You could in theory just copy the C/C++ addin dll from a Mac/Linux machine, or build it from source on Windows, and drop it into your Xamarin Studio installation. And if you contribute fixes to make it work well on Windows it will be shipped in future XS releases :)