var doc = app.activeDocument;

function layerpick(){
 var layer = doc.layers
  for(i=0;i<layer.length;i++){
   alert(doc.layers[i].name)
    if(doc.layers[i].typename=='LayerSet'){
      for(j=0;j<doc.layers[i].layers.length;j++){
       alert(doc.layers[i].layers[j].name)
      }}
    }
}

layerpick();