main is the starting point for an application; applets have a couple of automatically called methods: init, start, stop, destroy. You could put the PLAF code in a method and call it from both main and init.
getSystemLookAndFeelClassName() returns the Metal classname if it can't find a System one. Whether a system LAF is available depends on your Java install, maybe. Try UIManager.getAuxiliaryLookAndFeels() (returns an array), then getName on each of the LookAndFeel objects in the array. Just to see what LAFs you are getting back?