Ignore the advice to use KPPP - that's just ridiculous.
What you do need to do is make sure that the driver is loading correctly and the device is available. You can check for messages in 'dmesg' (dmesg|less) and you can also look for ttyUSB* somewhere in /dev (either /dev or /dev/USB).
Once you have established that the device driver is loaded and working, the next bit is to configure pppd - Xandros users will be able to give you better information on how to do that for Xandros.
For more information on configuring pppd, see:
http://tldp.org/HOWTO/PPP-HOWTO/c143.html#AEN145