Referencing a .class file in IntelliJ Java Project

Open the project structure dialog with "File->Project Structure", select "Project Settings->Modules" on the left, then select the module whose classpath you want to modify. On the "Dependencies" tab on the right side of the dialog you can add the classpath root directory of your class file.

Of course the class file has to be in the right directory structure corresponding to the package of the class, otherwise it will not work.

Select the root of your project and press F4. Then you'll see the project structure. Select the 'Libraries' section on the left hand side and click the plus symbol to create a 'New Project Library'. Choose 'Java' and select the directory where the .class file(s) is located. Proceed the remaining steps by clicking 'OK'.