This script is written in C#, my guess is that you might have saved it as a javascript. Also, there are some major errors in the script; here's a couple things to keep in mind when using C# in the Unity engine:

• You need to put using UnityEngine; and using System.Collections; at the beginning of your script.
• You need to put the rest of your code inside public class ScriptName : MonoBehaviour { } ScriptName being the name of your script (e.g. ScriptName.cs) (this is important!)

Here is the working script:

using UnityEngine;
using System.Collections;

public class NewBehaviourScript : MonoBehaviour {

    public Transform[] ObjectsToSpawn;
    public float SpawnInterval= 15;
    public int MaxObjectsSpawned = 8;
    public int MinObjectsSpawned = 3;
    public Vector3 minimum;
    public Vector3 maximum;
    public Vector3 center;

    private float _NextSpawn;

    void Start () { 
        _NextSpawn = Time.time + SpawnInterval;
        center = minimum - maximum;
    }

    void Update () {
        if(Time.time >= _NextSpawn) {
            int objectsToSpawn = Random.Range(MinObjectsSpawned, MaxObjectsSpawned);
            for (int i = 0; i < objectsToSpawn; i++) {
                //Instantiate a random object from the array ObjectsToSpawn within a minimum and maximum area which can be set in the inspector
                Instantiate(ObjectsToSpawn[Random.Range(0, ObjectsToSpawn.Length)], new Vector3(Random.Range(minimum.x,maximum.x), Random.Range(minimum.y,maximum.y), Random.Range(minimum.z,maximum.z)), Quaternion.LookRotation(center));
            }
            _NextSpawn = Time.time + SpawnInterval;
        }
    }

}