Wavelab Elements doesn’t include the Steinberg’s De-esser by default, but only the full version of Wavelab 8/8.5. If you have a VST plug-in from a third-party manufacturer, you might need to specify the path of the .dll file in Wavelab Elements (Options → Plug-in settings). Hit the “Force plug-in detection at next launch”, click on “OK” and restart the software.