The
easiest way to start debugging is to choose Debug 
Stop
Once from the VLISP menu. When this item is selected, the evaluation
of the first LISP expression will be interrupted. After that you
can resume program execution using various Debugger commands. Another
way to enter into the debugger mode is to set a breakpoint, as shown
in Setting a Breakpoint to Interrupt Program Execution.
When a break occurs, the corresponding VLISP text editor window will show the current LISP expression at the point which the break took place. A break loop marker will appear in the Console window. Using the Console window, you can access and manipulate the program environment in which the break occurred. You can also examine variables using the Watch window.