Ok well if the 3d object resource is a an obj. or .fbx etc then the game won't be able to work out what you need from it. Instead instantiate a new GameObject then add a component of type Mesh, loading your 3d resource into the Mesh component.
using UnityEngine;
using System.Collections;
public class NewBehaviourScript : MonoBehaviour {
private GameObject gameObject;
private Mesh myMesh;
void Start ()
{
gameObject = new GameObject("Empty");
gameObject.AddComponent<MeshFilter>();
gameObject.AddComponent<MeshRenderer>();
myMesh = (Mesh)Resources.Load("path",typeof(Mesh));
GetComponent<MeshFilter>().mesh = mesh;
}
You will end up with what you seem to be going for. Code not tested but the concept is there.
Oh and the path at run-time is relative to the resources folder so unless it is in Resources folder you'll need to navigate to it. example:
string path = "Models/mesh1";
Hope it helps.