Application.LoadLevelAdditive文档中说：
LoadLevel and LoadLevelAdditive don‘t happen immediately - a new level is loaded after the current game frame finishes. isLoadingLevel returns true if a level load was requested this frame already.
即LoadLevelAdditive到帧末才完成，所以要想获得新load进来的场景中的节点，不能立即find，可用Application.isLoadingLevel来判断load是否完成。如下所示：
Application.LoadLevelAdditive (sceneName);
while(Application.isLoadingLevel==true){
yield return null;
}//loading done
Transform sceneRoot = GameObject.Find(sceneRootName);
unity, LoadLevelAdditive到帧末才完成
原文：http://www.cnblogs.com/wantnon/p/5617223.html