System.IO.Packaging

For a C# solution in Visual Studio 2010 with .NET 4.0:

1. In your project's Solution Explorer, right-click on References and select Add References from the context menu.
2. Select Assemblies in the left-hand pane, then click the Browse button next to the File name field near the botton of the pane.
3. Browse to .NET 4.0 reference assemblies and select WindowsBase.dll. For example, on my machine (Windows 7, 64-bit) the complete path is:

C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.0\WindowsBase.dll

Save the solution (especially if you're compiling from the command-line with MSBuild) and you should now be able to add the using System.IO.Packaging directive to the top of your .cs file without an error appearing.

According to a user comment on this MSDN page, you have to add a reference to the WindowsBase .Net library.