How to add System.Windows.Interactivity to project?

The easiest way might be to get it from NuGet:

http://www.nuget.org/packages/System.Windows.Interactivity.WPF/

Although this issue is quite old, i think this is relevant news / the most recent answer: Microsoft open-sourced XAML Behaviours and posted a blog post how to update to this version: https://devblogs.microsoft.com/dotnet/open-sourcing-xaml-behaviors-for-wpf/

To save you a click, this is the main steps to migrate:

1. Remove reference to “Microsoft.Expression.Interactions” and “System.Windows.Interactivity”
2. Install the Microsoft.Xaml.Behaviors.Wpf NuGet package.
3. XAML files – replace the xmlns namespaces http://schemas.microsoft.com/expression/2010/interactivity and http://schemas.microsoft.com/expression/2010/interactions with http://schemas.microsoft.com/xaml/behaviors
4. C# files – replace the usings in c# files “Microsoft.Xaml.Interactivity” and “Microsoft.Xaml.Interactions” with “Microsoft.Xaml.Behaviors”

There is a new NuGet package that contains the System.Windows.Interactivity.dll that is compatible with:

• WPF 4.0, 4.5
• Silverligt 4.0, 5.0
• Windows Phone 7.1, 8.0
• Windows Store 8, 8.1

To install Expression.Blend.Sdk, run the following command in the Package Manager Console

PM> Install-Package Expression.Blend.Sdk

http://www.nuget.org/packages/Expression.Blend.Sdk/

The official package for behaviors is Microsoft.Xaml.Behaviors.Wpf.

It used to be in the Blend SDK (deprecated).
See Jan's answer for more details if you need to migrate.