What exactly do mean with "start"?
Should everything be there immediately and when pressing enter, objects will start moving etc. (like pressing 'run' when hitting enter)?
Do you want to have nothing at the start and when pressing enter, the world is created and starts running?
Your scenario must already be running and the code must be in an act method (or a method it calls) of a currently active object (either the currently active world or an actor that is currently in that world) .