I found the solution: you need to add 'Source' to the "get_field" function inside "qa-wysiwyg-editor.php".
For example:
$qa_content['script_lines'][]=array(
"qa_wysiwyg_editor_config={toolbar:[".
"['Bold','Italic','Underline','Strike'],".
"['Font','FontSize'],".
"['TextColor','BGColor'],".
"['Link','Unlink'],".
"'/',".
"['JustifyLeft','JustifyCenter','JustifyRight','JustifyBlock'],".
"['NumberedList','BulletedList','-','Outdent','Indent','Blockquote'],".
"['Image','Flash','Table','HorizontalRule','Smiley','SpecialChar'],".
"['RemoveFormat', 'Maximize', 'Source']".