While creating a new Dockable Window add-in for 10.1, I see there is a WPF designer type. After choosing this, I created a simple UI. The UI displays correctly however when I add a reference to System.Windows.Interactivity, the Add-In no longer loads. When I attempt to debug the add-in, I am getting the "Could not load file or assembly 'System.Windows.Interactivity" message. which is funny because Copy Local is set to true for the reference and I verified it is there.