There are a couple ways to get to the image plane node. The easiest is to open the outliner and under display check the shapes box. Then select the camera and under the camera you will find the image plane and image plane shape nodes.
But it's a bit more convoluted then simply having the image plane input node show up in the channel box.
I am also using 2013.
Well I checked this on another system that still has 2012 on it and the input for the camera node shows up.
So this currently looks like a new 2013 behavior where the input node for the camera does not appear in the channel box.
I rarely use image planes so I never noticed it until now.
Maybe someone else knows if there is a secret handshake in maya 2013 to turn this back on.
"If I have seen further it is by standing on the shoulders of giants." Sir Isaac Newton, 1675
Last edited by ctbram; 31-12-2012 at 06:48 PM.