Creating an Add-in Project
To create a new SharePoint Workspace 2010 Add-In project in Visual Studio 2008:
Select File\New\Project…
Select Visual C# (not any of its subprojects) and then "SharePoint Workspace 2010 Add-In" from My Templates.
The project defines an Add-in class that implements the following interfaces:
UserControl
ISPWManagedToolAddin
ISPWManagedToolAddinUI
IRibbonExtensibility