void Start () {
UISlicedSprite sp1 = NGUITools.AddWidget<UISlicedSprite>(GameObject.Find("Panel")); //this will likely give you problems since you may have more than a single "Panel" object.
//slight rework
GameObject atlasPrefab = Resources.Load("inventory") as GameObject;
UIAtlas atlas = Instantiate(atlasPrefab).GetComponent<UIAtlas>();
sp1.atlas = atlas;
sp1.spriteName = "item_A_Tr";
}