These movements have rotors that are press fitted on to the movement. In order to reattach, simply align the rotor and press it down.
If your rotor is already loose, it may be best to take this as a sign that the pressure fit needs to be reinforced. To do this, place a tab or two of GS Hypocement and let the rotor dry over night. After that, seal that baby up and your rotor issues will be solved.