I can't run simple movement script from "Space Shooter".

I am a complete beginner with game development and script. I'm trying to work through the "Space Shooter" beginner project from the unity site, and I can't get the script from the video tutorials to work.

This is the script:

using UnityEngine;

using System.Collections;

public class Player_Controller : MonoBehaviour

 {
     void fixedUpdate ()
     {
         float moveHorizontal = Input.GetAxis ("Horizontal");
         float moveVertical = Input.GetAxis ("Vertical");

         Vector3 movement = new Vector3 (moveHorizontal, 0.0f, moveVertical);
         rigidbody.velocity = movement;
     }
 }

The correct keyboard keys are set in the input manager and it comes up with no error in the console or footer, but where it is added as a component to the player object, it says "The associated script cannot be loaded. Please fix any compile errors and assign a valid script.