Properties does not exist: System.Windows.Interactivity?
If it worked at runtime but not design-time then I am blaming VS 2010 for being buggy. I've had VS 2010 not recognize the local namespace when imported into XAML, but when opened in VS 2012 it worked in the designer with no problems. Unfortunately, Visual Studio 2010's XAML/WPF support is far from perfect. Expression Blend or Visual Studio 2012 are better.
I did like ViktorLaCroix 's suggestion. I would try
xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"Instead of importing it using clr-namespace and see if that helps you. That's how I am using it and it works fine. I'll also show you my reference details for the library.
System.Windows.InteractivityC:\Program Files (x86)\Microsoft SDKs\Expression\Blend\.NETFramework\v4.0\Libraries\System.Windows.Interactivity.dllv4.0.30319
The easiest way might be to get it from NuGet:
To install System.Windows.Interactivity v4.0 for WPF, run the following command in the Package Manager Console
PM> Install-Package System.Windows.Interactivity.WPF
http://www.nuget.org/packages/System.Windows.Interactivity.WPF/