How do I use a C# Class Library in a project?

Add a reference to it in your project and a using clause at the top of the CS file where you want to use it.

Adding a reference:

1. In Visual Studio, click Project, and then Add Reference.
   
2. Click the Browse tab and locate the DLL you want to add a reference to.
   NOTE: Apparently using Browse is bad form if the DLL you want to use is in the same project. Instead, right-click the Project and then click Add Reference, then select the appropriate class from the Project tab:
   
3. Click OK.

Adding a using clause:

Add "using [namespace];" to the CS file where you want to reference your library. So, if the library you want to reference has a namespace called MyLibrary, add the following to the CS file:

using MyLibrary;

In the Solution Explorer window, right click the project you want to use your class library from and click the 'Add Reference' menu item. Then if the class library is in the same solution file, go to the projects tab and select it; if it's not in the same tab, you can go to the Browse tab and find it that way.

Then you can use anything in that assembly.

I'm not certain why everyone is claiming that you need a using statement at the top of your file, as this is entirely unnecessary.

Right-click on the "References" folder in your project and select "Add Reference". If your new class library is a project in the same solution, select the "Project" tab and pick the project. If the new library is NOT in the same solution, click the "Browse" tab and find the .dll for your new project.