The best way to add reference to Moq framework is installing it from Nuget. Also you still candownload Moq.dll and add reference to this library (usually I create folder
libs under the solution folder, where I put all third-party libraries, which is not available via Nuget).
BTW Another option to install package from Nuget - right click on project references and select
Manage Nuget packages.... Then search online for Moq and install it. See why use Nuget over installing libraries directly on my machine