Where is the System.Windows.Forms.Integration namespace?

You'll need to add a reference to WindowsFormsIntegration. If you do a Add Reference in Visual Studio, it will be near the last DLL available in the .Net tab

On my machine it's located at

%ProgramFiles%\Reference Assemblies\Microsoft\Framework\v3.0\WindowsFormsIntegration.dll

Visual Studio filters the available namespaces according to the type of project you chose. If you need a namespace that is not listed, click the [Add...] button on the references tab of your project's properties. This will pop-up the Reference Manager window. Choose "Framework" from the list of Assemblies in the left-hand panel. This will display all the Microsoft provided namespaces. You will find WindowsFormsIntegration there.

Just came across the same problem.

While adding a reference to WindowsFormsIntegration.dll didn't work as this has no WF controls in it, I did find that adding a reference to System.Windows.Forms worked perfectly fine.