In order to use an Event Handler, you need to first place an Event Module in the Particle Update group of the emitter that is generating the event. For example, if you want the particles in Emitter B to follow the particles in Emitter A, you would place a Generate Location Event module in the Particle Update group of Emitter A. Then you would add an Event Handler to Emitter B, along with a Receive Location Event item, to listen for that location event.