VLISP provides a preview feature for checking the results of your DCL coding.
In this case, your DCL file defines just a single dialog box, so there is no choice to be made. As you create larger and more robust applications, however, you may end up with DCL files containing multiple dialog boxes. This is where you can select which one to preview.
VLISP passes control to AutoCAD to display the dialog box. If AutoCAD finds syntactical errors, it displays one or more message windows identifying the errors.
If AutoCAD detects DCL errors and you are unable to figure out how to fix them, copy the gpdialog.dcl file in your Tutorial\VisualLISP\Lesson4 directory and save it in the Support directory.