The site says this:
NOTE: RhinoCommon is the new .NET plug-in SDK for Rhino 5.
Rhino.NET will NOT be available in Rhino 6. Use RhinoCommon if possible
The Rhino.NET SDK is designed to allow .NET developers to create Rhino plug-ins with the same capabilities as Rhino C++ SDK developers.
.NET Framework Plug-In Development (Rhino 4 and 5)
Rhino ships with the necessary components to enable Rhino.NET plug-ins on users computers. Plug-ins can be created with a compiler that compiles assemblies using .NET 2.0, including:
Microsoft Visual C# and Microsoft Visual VB.NET 2005 and 2008 (including Microsoft Visual Studio Express editions)
Microsoft Visual C# and Microsoft Visual VB.NET 2010 are supported on Rhino 5
SharpDevelop 2.0 and above
What is the Rhino.NET SDK?
Rhino.Net SDK helps developers to extend Rhino’s functionality using ‘Rhino plug-ins’. The Rhino.NET SDK provides the tools to develop Rhino plug-ins using a .NET programming language. This means that Rhino plug-ins can now be developed using VB.NET, C#, C++.NET, Delphi.NET (and the list goes on).
The Rhino.NET SDK is actually a wrapper around the Rhino C++ SDK. Most of the classes and methods provided by the C++ SDK are available through .NET.
Where is the Rhino.NET SDK?
The Rhino.NET SDK an assembly named Rhino_DotNET.dll located in the Rhino system directory. The Rhino_DotNET.dll assembly is what all Rhino.NET plug-ins reference to get exported classes and functions from Rhino. This file is installed with Rhino and is not a separate download.
Post questions or read other people’s questions/answers at http://discourse.mcneel.com/
Some of your questions may be answered by the DotNET Essentials page
But if you open Microsoft Visual Studio Express you can’t imediately start making rhino plugins because you need to install rhinocommon/rhino sdk. Where do you download this?
I can find it now easily but when I started it was a bit of searching.
A direct link to the download page for the template would be easier