How do I make an array and then access an Object from the array

It looks like you're doing everything right as far as the code goes. You just have a null reference, so that means that something is not being found. You seem to know where your errors are, so that should narrow down your problem quite a bit.

If I were debugging this, I'd first make sure that there is a GameObject called MainCamera childed to "Player".

But since I'm not, I'd also suggest creating a public field in your script for you to just drag the camera onto the inspector, like so:

     public Camera playerCamera;

Now checkout the inspector for your Player, and you should see the field. Drag in the camera, and test it.

For your "ControllableObjects" problem, you probably can't just drag in a bunch of references, because, I assume, they're always changing. Again, it seems like the problem is that your gameObject doesn't have a child named "MainCamera". There really isn't anything else in your code to go wrong at that point.

Nevermind, I fixed it. Thank you though...

 var canUse                        : boolean    = true;
 private    var controllableObjects    : GameObject[];
 private    var playerObject        : GameObject;
 private    var playerCamera        : GameObject;
 private    var targetObject        : GameObject;
 private    var targetCamera        : GameObject;
 var rayDepth                    : float        = 3;
 
 class ControlObjectManager extends MonoBehaviour
 {
     function Start ()
     {
         // Add the 'Player'
         playerObject = GameObject.FindGameObjectWithTag("Player").gameObject;
         playerCamera = GetChildByTag(playerObject, "Camera", "MainCamera");
         
         controllableObjects   = GameObject.FindGameObjectsWithTag("ControllableObject");
         
         // iterate through the array
         for (var obj in controllableObjects)
         {
             if (obj.tag == "ControllableObject")
             {
                 obj.SendMessage ("SetControllable", false);
                 var objCamera : GameObject = GetChildByTag(obj, "Camera", "MainCamera");
                 
                 if (objCamera)
                 {
                     objCamera.camera.depth = 1;
                     objCamera.camera.active = false;
                 }
                 else Debug.Log("No Camera Attached To : " + obj + " : Tagged : " + "MainCamera");
             }
         }
     }
     
     function Update ()
     {
         if (canUse && Input.GetButtonDown("Use"))
         {
             //Debug.Log("Use Activated !");
             
             if (!targetObject)
             {
                 var camX    = playerCamera.camera.GetScreenWidth()    / 2;
                 var camY    = playerCamera.camera.GetScreenHeight()    / 2;
                 
                 var hit: RaycastHit;
                 var ray = playerCamera.camera.ScreenPointToRay (Vector3(camX, camY, rayDepth));
                 
                 if (Physics.Raycast(ray, hit) && hit.collider != null && hit.collider.gameObject.tag == "ControlTrigger")
                 {
                     playerObject.SendMessage ("SetControllable", false);
                     playerCamera.camera.active = false;
                     playerCamera.camera.depth = 1;
                     
                     targetObject = hit.collider.gameObject.GetComponent(ControlTrigger).controlObject;
                     targetObject.SendMessage ("SetControllable", true);
                     
                     targetCamera = GetChildByTag(targetObject, "Camera", "MainCamera");
                     targetCamera.camera.active = true;
                     targetCamera.camera.depth = 0;
                 }
                 else
                 {
                     targetObject = null;
                 }
             }
             else
             {
                 playerObject.SendMessage ("SetControllable", true);
                 playerCamera.camera.active = true;
                 playerCamera.camera.depth = 0;
                 
                 targetObject.SendMessage ("SetControllable", false);
                 
                 targetCamera.camera.depth = 1;
                 targetCamera.camera.active = false;
                 targetObject = null;
             }
         }
     }
     
     function GetChildByTag(searchObj:GameObject, withType:String, withTag:String)
     {
         switch (withType)
         {
             case "GameObject" :
                 var childObjects = searchObj.transform.GetComponentsInChildren(GameObject, true);
                 
                 // iterate through the array
                 for (var obj in childObjects)
                 {
                     if (obj.gameObject.tag == withTag) return obj.gameObject;
                 }
             break;
         
             
             case "Camera" :
                 var childCameras = searchObj.transform.GetComponentsInChildren(Camera, true);
                 
                 // iterate through the array
                 for (var cam in childCameras)
                 {
                     if (cam.gameObject.tag == withTag) return cam.gameObject;
                 }
             break;
         }
     }
 }