To get the tools you need to build systems, you'll need to clone our git repository from CodePlex.
You'll also need Visual Studio 2012. Once you have this installed, you can install the Dafny extension from the git repository as follows: With Visual Studio not running, launch tools/Dafny/DafnyLanguageService.vsix, then start Visual Studio.
Note: if you already have the Dafny extension installed, this process isn't sufficient. You have to first uninstall it from Visual Studio by clicking Tools->Extensions and Updates, then selecting DafnyLanguageMode from the central panel, then
clicking Uninstall, then shutting down Visual Studio. After that, you can launch the .vsix file to install.