You need to reference the Microsoft PresentationFramework assembly because you are trying to use WPF namespace .
If you created a WPF project from scratch, it must be enabled. When you start in VS with some other type of project, you may need to add this link manually. Right-click the node links in your project, add the Add Link command. Then, in the dialog that appears, you go to the .NET tab and view the assembly you want.