Callback functions are not listed in libraries: you write the callbacks for controls on your panels, so they are specific to your program. Your callback may call any number of library functions.
Library functions can be easily browsed in the CVI environment. Select Library from the menu in the project window or any code window. You can then browse functions based on category or function name. When you select a function, a new window opens for the CVI function panel. Using the function panel, you can build the function call which you can insert into your program using the Insert Function Call button or option under the Code menu.