The code works for me. It has to be called every single frame of course. Take the car example that comes with jPCT and put this
camera.setBack(car.getWorldTransformation().invert3x3());
camera.setPosition(car.getTransformedCenter());
at the very end of the moveCamera()-method and you'll see what it's supposed to do. If you are calling it every frame and it still doesn't work, please provide me with a test case that demonstrates the problem, so i see what's wrong.
About the addChild(<Camera>)...no, that wouldn't be cool, because it makes the camera part of the scene graph, which it shouldn't be by design. If you want that, do it yourself. The code above should do it. If it doesn't, well...then please provide me with the mentioned test case, so i can check this out.