How can I do it? I know it should be a basic thing but I didn't find it in the manual............

Ketron keyboards do not have a polarity change function like some keyboards. You either have to go and buy a different pedal or go into the pedal and modify the wires in reverse. One of the things I like about the Roland Sustain pedals is that if you open it up you will see two poles you can solder the hot wire to. One says "+" and the other says "-". This way you can easily change the wire to accomodate for Yamaha/Roland or Korg/Ketron, etc. keybaords.
George Kaye
Kaye's Music Scene
Reseda, California