Why can't I use this Namespace in a Standard WinForms applicaiton.

It is available in WPF, and so I thought I'd try it in a WinForm app but it isn't visible. The Reflector says that the namespace is included in WindowsBase ver 3.0.0.0 which is the version used in my WinForm application, so why can't i use the System.IO.Packaging.ZipPackage class?

__________________
Jaeden "Sifo Dyas" al'Raec Ruiner
http://www.wayoftheleaf.net/