Instantiate GameObject on Enum Selection from Inspector

This will be a pretty basic example to get you started. You can accomplish this in a few different ways, but the most educational seemed to be an introduction to custom inspectors. Create a script called PrimativeSpawner, paste this in:

 using UnityEngine;
 using System.Collections;
 using UnityEditor;
 
 public class PrimativeSpawner : MonoBehaviour {
 
     // Active primative choice
     public UnityEngine.PrimitiveType primativeChoice;
 
     // Spawn a primative based on the enum choice
     public void CreateChosenPrimative()
     {
         // Check what's currently active
         GameObject newPrimative  = GameObject.CreatePrimitive(primativeChoice);
 
         // Print the new thing's name
         Debug.LogFormat("New Primative created!  {0}", newPrimative.name);
     }
 }
 
 // We can use a custom  inspector  for this
 [CustomEditor(typeof(PrimativeSpawner))]
 public class PrimativeSpawnerInspector : Editor
 {
     // Keep track of our current object being  inspected
     public PrimativeSpawner spawner;
 
     // Replace the given inspector gui
     public override void OnInspectorGUI()
     {
         // Draw the given inspector
         this.DrawDefaultInspector();
 
         // Check that it's been initialized
         if (spawner == null)
             spawner = this.serializedObject.targetObject as PrimativeSpawner;
 
         // Simple Button
         if (GUILayout.Button("Spawn Primative"))
             spawner.CreateChosenPrimative();
     }
 }

You'll be able to choose a prefab then create it with a button click in its inspector: