This modifier use a Camera
present in the composition in order to do an UV mapping over any object.
In the case of the other mapping tool, the UV Mapping
the camera is embedded to the modifier, here it actually is in the scene.
How to use the camera mapping modifier
This modifier, as say it's name is so the one used for camera Mapping.
To use this modifier, first apply it to the surface you want to map, then select your projection camera.
When it's done, simply put your projected image inside of the geometry renderer :
Simple as that !
Note that if you are importing a scene from an .fbx, .blend or .3ds your camera position and orientation should be well imported (if you crushed all transformation before the export) but you may have to verify the Horizontal FOV of the projection camera :
The camera will probably be imported with a "matrix" placement, so do not forget to change it back to "target, orientation, distance" which is Camera default placement :
If you really want to see it with your eyes, here is a tutorial based on one of the examples :