How do I activate the Control Lock feature to prevent children from changing settings?

Press and hold the CONTROL LOCK button for 5 seconds to activate or deactivate the controls. When Control Lock is set, all buttons are disabled except the ON/OFF button, and the Control Lock indicator will illuminate while 'CL' is displayed alternately with the estimated time remaining. To unlock the controls, press and hold the CONTROL LOCK button again for 5 seconds. (Page 25)