At the top left of the debugger window you'll find a text field where you can enter JavaScript code. This code will be executed in the context of the currently displayed web page when you click the "Execute" button (located at the right side of the window).
In the bottom part of the window iCab will display the result of the code that is executed (if there's any result). Also you can use the JavaScript function
debug(expression)in the JavaScript code to print the result of the expression in the bottom part of the window. This way you can easily print strings or inspect the content of variables etc.
The button Delete input deletes the text field at the top left with the code, the button Delete log deletes the bottom part of the window.