Error Attempting to Include Library/Class in C# VS 2008 Team Edition

I am attempting to create a ContextMenuEventArgs variable to pass into an event handler function. When I attempt to include the library "System.Windows.Controls", I end up with the error message "The type or namespace name 'Controls' does not exist in the namespace 'System.Windows' (are you missing an assembly reference?)" The code to include the library is:
using System.Windows.Controls;

I am using Visual Studio 2008 Team Edition, and I have verified that the library should have been installed with the .NET 3.0 and higher framework. Could someone help me out with this error, as I do not see anything wrong with the code that would cause this issue? Thank you in advance for your help.