Summer Infant Slim Secure Baby Video Monitor "extra second" Questions

How do I add an extra camera to this monitor? I have purchased an extra camera for this monitor but cannot get the hand held unit to recognise the camera. First you have to select your prefered camera

...extra camera to this baby monitor? you need to power on the monitor. second, push the power button so you see the list of camera numbers at the bottom. third, scroll over to the camera of choice, ...