Till 13.10 Xubuntu uses XScreenSaver as screensaver. In the rare case you installed xfce manually you may want to use
sudo apt-get install xfce4-settings
from the universe package to be able to use the settings manager.
Light-locker and xscreensaver – to have both is not a good idea see BUG 1304128.
I personally prefer xscreensaver:
# apt-get remove light-locker # apt-get install xscreensaver xfishtank xdaliclock