Under Ubuntu 7.04 (Feisty), you can configure the time set for launch of screensavers but not the screensavers itself.
Below is a suggesting way of how to troubleshoot this setting:
Download and install the xFx ScreenSaver settings on below link:
Launch the settings xfx-screensaver-settings
The settings of the screensaver can be modified as from xfx by clicking on "Settings
If you want more screensavers, install all the packages starting with xscreensaver (run Synaptic to retrieve them).