Thanks qlob.
I did so and now I'm a step further but not done yet, because I have a white screen as a screensaver. Can it be because the last book that I opened is a mobi and this feature would only works with azw books (which I don't have) ?
***EDIT*** Forget this message, now it works perfectly. I don't know why it needed 5 minutes or so for the screensaver to appear, but ok, it's there.