System.Windows.Control.Toolkit missing

Trying to use Expander, seems like I need to add System.windows.controls.toolkit as a reference. However, I am not able to locate anywhere in the Microsoft SDK folder, where it is suppose to be?

I have Visual Studio 2010 and Silverlight 4.0 SDK installed.

You need to install the silverlight toolkit... http://silverlight.codeplex.com/