The Adafruit Class Library is a special library package containing Windows IoT Core driver software for a variety of Adafruit products. To use the library, you must add a reference to it in your project. 

To add the reference to the Adafruit Class Library, you'll need to use the NuGet Package Manager, which is a standard part of Visual Studio.

To get to the Package Manager, open the Project Menu and select "Manage NuGet Packages..."

In the Package Manager window, select "Browse", and enter "AdafruitClassLibrary" in the search box. Select the library in the list, and click the Install box on the right-hand side of the window

You should now see AdafruitClassLibrary under References in Solution Explorer.  That's all there is to it!