The tutorial is self-explanatory. Take the time to read the example in their entirety. Don’t just skim over it.
You add a mapping
inputManager.addMapping("WhateverNameSuitsYou", new MouseButtonTrigger(MouseInput.BUTTON_LEFT));
Then you add this to the proper listener (action or analog)
inputManager.addListener(actionListener, new String[]{"WhateverNameSuitsYou"});
Finally in the actionListener you tell jME3 what to do when that key is triggered.
private ActionListener actionListener = new ActionListener() {
public void onAction(String name, boolean keyPressed, float tpf) {
if (name.equals("WhateverNameSuitsYou") && !keyPressed) {
isRunning = !isRunning;
}
}
};