When adding folders in the visual studio, you may notice items of folder not being included by Visual studio. You can easily include the files items by doing the following thing.

In your Solution Explorer of the current project, there is an icon called Show All Files. If you click this icon, in your Solution Explorer tree should display all files or folders that are not part of the solution. It will be grayed white color. The images below is taken from Visual Studio 2015.

If you want to include files or folder, simply right click the item and choose the option Include In Project.