Two posted questions, one response...

JBeck
I installed my Knoppix to the hard drive, and I have the "Lock Screen" enabled - the problem with not being able to run the Lock Screen, is that you dont have a password that will be used to "unlock" the screen. What you might try, is set a "user" password, and while you are at it, set a "root" password - as a test - and then see if the "Lock Screen" is enabled - you NEVER want to enable the Lock Screen without having a password, you will not have a way to disable the locked screen...

AlanEzust
I am using xscreensaver, not the KDE one, but, my command may be translated to yours, to fire-off my screensaver, the "X" one, I just run xscreensaver-command -activate, and if I wanted to lock the screen at the same time, I would add -lock to my command line. Not sure on this, I cant locate any information ( man or other ), but I think the program is called kscreensaver, I did a locate on it, and can only find the copyright docs for it, not the program. Best I can do for you...

Hope this helps, both of you,
Ms. Cuddles