When I had mine (i use AP racks now, but the terrestrial cages worked great!) I actually placed the probe on the heat cable right at the point where it comes out of the back of the cage. You'll see when you get yours. That's the place the worked for me. AP is right in saying to attach the probe somewhere outside of the cage. I found that securing the probe somewhere inside the cage was a pain in the butt and I found another way.

You'll need a reliable digital thermometer to do it right. When I attached the probe directly to the heat cable, I ran the heat cable at about 96, as indicated by the thermostat. The other thermometer gave me a reading of 92 inside the cage.

You'll "get it" a little more when you can see the cage in front of you and visualize things a little better. Get back to us then and we'll get you set up right!