There are two ways to add an eventHander. One is by using the menu. Choose File-Scripts-Scripts Event Manager... then set it up in the dialog that opens.
I prefer to install with a sperate script.
if(!app.notifiersEnabled) app.notifiersEnabled = true;
var hasSelectEvent = false;
for(var e = 0;e<app.notifiers.length;e++){
if(app.notifiers[e].event == 'slct') hasSelectEvent = true;
}
if(!hasSelectEvent) {
var eventFile = new File('~/desktop/layerPreset.jsx');
app.notifiers.add( "slct", eventFile);
}