In newer versions of Ubuntu (and Linuxes) xscreensaver has been replaced with gnome-screensaver. The same screensavers are supported (with some exceptions, I guess), but there is one thing that is lacking: per-screensaver configuration. The author of gnome-screensaver thinks that this is not needed.
There are some ways to get around this, among them: replace gnome-screensaver with xscreensaver (this has some side-effects that may not be desired) or install xscreensaver alongside gnome-screensaver.
If you are comfortable with editing configuration files and reading man pages I would recommend the following way: find and edit the desktop file of each screensaver you want to configure, using the screensaver’s command line options.