Where to find "Microsoft.VisualStudio.TestTools.UnitTesting" missing dll?

I know this is old, this is what came up in my Google search. I needed to reference these packages on NuGet:

You have to add reference to

Microsoft.VisualStudio.QualityTools.UnitTestFramework.dll 

It can be found at C:\Program Files\Microsoft Visual Studio 10.0\Common7\IDE\PublicAssemblies\ directory (for VS2010 professional or above; .NET Framework 4.0).

or right click on your project and select: Add Reference... > .NET:

The DLL you're looking for that contains that namespace is

Microsoft.VisualStudio.QualityTools.UnitTestFramework.dll

Note that unit testing cannot be used in Visual Studio Express.

To resolve this issue, I had to do the following:

1. Launch the Visual Studio Installer with administrative privileges
2. If it prompts you to install updates to Visual Studio, do so before continuing
3. When prompted, click the button to Modify the existing installation
4. Click on the "Individual components" tab / header along the top
5. Scroll down to the "Debugging and testing" section
6. Check the box next to "Web performance and load testing tools"
7. Click the Modify button on the bottom right corner of the dialog to install the missing DLLs

Once the DLLs are installed, you can add references to them using the method that Agent007 indicated in his answer.