The project you have downloaded compiles into a dll assembly and provide a set of classes with implemented functionality.
You should add to your solution a new project with Output Type of either Console Application or Windows Application (VS Add Project wizard will offer you different templates of Projects).
In the newly added project, you can implement logic to test your Class Library.
Output type of the project you can find and change by the following steps:
Right click on project in Solution Explorer -> Properties.
In opened tab with properties select Application and there will be ComboBox marked with Output Type label.
Just right click on the Project Solution
A window pops up.
Expand the common Properties.
Select Start Up Project
In there on right hand side Select radio button with Single Startup Project
Select your Project in there and apply.
That’s it. Now save and build your project. Run the project to see the output.