Nuget is an open source package manager for Visual Studio 2010 and Visual 2012 that can download packages and manage dependencies. It is included by default in VS 2012 but VS 2010 users will need this extension.
To download and install the 51degrees package you will need to access the Nuget gallery via a project's context menu. Right click on the project (not the solution) and select 'Manage Nuget Packages...'
This opens the galley dialog. Search 51Degrees and press the Install button on the 51Degrees package (not the WebMatrix package).
You should then see Nuget installing the package and dependencies. Finally you will see that your project has had the 51Degrees references added and some sample configuration files.