Where did System.Design go?

I am making a C# project in which I am using ScintillaNet, and it says:

The referenced assembly "ScintillaNet" could not be resolved because it has a dependency on "System.Design, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" which is not in the currently targeted framework ".NETFramework,Version=v4.0,Profile=Client". Please remove references to assemblies not in the targeted framework or consider retargeting your project.

I tried adding a reference to System.Design, but it doesn't exist in my list. Do I need to download it some开发者_如何学Gowhere? I have Microsoft Visual Studio 10.

Go to the project's properties and select the target framework to be .NET 4.0, not Client Profile