Using a custom renderer is only half of the solution. A custom renderer will break the default items selection by using the keyboard of a combo box. See Combo Box With Custom Renderer for more information and a more complete solution.
The main problem with your code is the NotWorking class. This class should NOT extend JPanel. It is just a class that is used to hold the 3 properties of the class. The course number, title and level should NOT be static variables. There should be no reference to Swing components.
Your design should be one class for the NotWorking class and another class to create the GUI.
Start with the section from the Swing tutorial on How to Use Combo Boxes for a better design. Then customize the ComboBoxDemo.java
example code from the tutorial to add your NotWorking class to the combo box instead of adding String data.