I have my project set to .NET Framework 4.0. When I add System.IO.Packaging, it says that it doesn\'t exist. It also doesn\'t show up when I try to add it as a
System.IO.Packaging
You need to add a reference to the WindowsBase.dll. System.IO.Packaging is located in there.
See this article for more details:
http://msdn.microsoft.com/en-us/library/system.io.packaging.package.aspx