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 is a namespace, not a reference. Most (all?) of the classes within the namespace, such as ZipPackage, are deployed in WindowsBase.dll.
Make sure you have a reference to WindowsBase.dll - if you do, you can just add: using System.IO.Packaging; to your .cs files, and you'll be fine.
Note that you can see this in the documentation for any class on MSDN. For example, in ZipPackage, it lists:
Namespace: System.IO.Packaging
Assembly: WindowsBase (in WindowsBase.dll)