Installing and Referencing the Outlook 2007 PIA
You must first install the Outlook 2007 Primary Interop Assembly (PIA) in the Global Assembly Cache (GAC) before you can incorporate information from the PIA in an Outlook managed add-in. You can do so by choosing the Typical installation of Office and then adding .NET Programmability Support for Outlook.
To add .NET Programmability Support for Outlook after Outlook is installed
In Control Panel, click Add or Remove Programs.
Select your installation of Office and click Change.
Select Add or Remove Features.
In the Installation Options tab, click the '+' to expand the application for which you want to install the PIA.
Click the drop-down arrow adjacent to .NET Programmability Support and select Run from My Computer.
After installation, if you are using Visual Studio to create the managed project, you can add a reference to the Microsoft Outlook 12.0 Object Library component. Subsequently, in the object browser, under the Microsoft.Office.Interop.Outlook namespace, you can see managed interfaces in the PIA that have names corresponding to objects in the Outlook object model.
For information about other ways to install the PIA for Office, see How to: Install Office Primary Interop Assemblies.