OpenXml: add reference to System.IO.Packaging

In order to use the API of OpenXML, I need to add a reference to "System.IO.Packaging" to the .Net project.

In a C# file, I cannot reference the .dll directly using "System.IO.Packaging".

Then, I  right click the Reference and then select "Add Reference..." under Solution Explorer 

But I cannot find "System.IO.Packaging" under Tab ".Net"

After my searching, I know that I need to add a reference to "WindowBase" instead of "System.IO.Packaging"

Then, I can reference successfully.