You either need to run REST Streaming or do this through IFTTT. With IFTTT, you would pick off the notifications received from Nest on your phone or Gmail (Android) and then trigger an Action in ST (turn on virtual switch / alarm) which your Piston then runs off of.
NST Manager alone will not pick up the motion events. Even though you might see the past camera image from a motion event inside the app under the camera device in Things, this is a different portion of the API that is used to pull the last image from Nest. The polling time (API limitation) between NST Manager and Nest is not enough to grab the motion event and record it in ST in time without the use of REST.