How to add an asset to a script-enabled public game object?

I’m following a tutorial that was created for Unity 4.6, and am up to a point where we are instantiating prefabs at runtime. The prefab to be created is added to the Inspector in Unity, with the codes public GameObject enemyPrefab and Instantiate (enemyPrefab, new Vector3 (0, 0, 0), Quaternion.identity);.The problem is that, with this being a GameObject, I’m unable to instantiate by using this unless an instance is already in the Hierarchy. So how do I add an asset to the position in the Inspector without adding it to the Hierarchy first?

I think what Hex is getting at ^^
Put your script that calls the object to instantiate on an empty gameobject in the scene. Even if it has no renderer, something like a “gamemanager”. Then use it to call the "instantiate(enemyPrefab) once the scene starts.
If you’re trying to “create an object from an instance” put a blank copy of your prefab with no renderer in the scene and see if that solves your problem?