Class definition not recognized after load level

General description: I just migrated my awesome game from unity 3.5 to 4.5. All objects seem to be ok. The game runs, but now it has some extrange behaviour.

Detalis: I have defined a couple of classes inside the Plugin folder, for instance this one:

 using UnityEngine;
 using System;
 using System.Data;
 using System.Collections;
 using System.Collections.Generic;
 
 public class MapsClass : MonoBehaviour {
 
 //Variables or attributes - Start
     public String MapName;
     public String MapLoadName;
     public Texture MapLoadTexture;
     public int MapMaxRunners;
     public int GoldCost;
     public int BronzeCost;
     public int ReqLevel;
     public int FirstPlacePrize;
     public int SecondPlacePrize;
     public int ThirdPlacePrize;
 
 //Variables or attributes - End
     public MapsClass (String mapName,String mapLoadName,Texture mapLoadTexture, int mapMaxRunners, int goldCost, int bronzeCost, int reqLevel, int firstPlacePrize, int secondPlacePrize, int thirdPlacePrize){
         //MapsClass    theMap = new MapsClass();
         MapName = mapName;
         MapLoadName = mapLoadName;        
         MapLoadTexture = mapLoadTexture;
         MapMaxRunners = mapMaxRunners;            
         GoldCost = goldCost;
         BronzeCost = bronzeCost;
         ReqLevel = reqLevel;
         FirstPlacePrize = firstPlacePrize;
         SecondPlacePrize = secondPlacePrize;
         ThirdPlacePrize = thirdPlacePrize;
         
         //return theMap;
     }
 
     public MapsClass(){
         MapName = "";
         MapLoadName = "";
         MapLoadTexture = null;
         MapMaxRunners = 0;
         GoldCost = 0;
         BronzeCost = 0;
         ReqLevel = 0;
         FirstPlacePrize = 0;
         SecondPlacePrize = 0;
         ThirdPlacePrize = 0;
     }
 }

Now, the class is referenced inside a normal JS which holds this instruction: DontDestroyOnLoad(transform.gameObject);

The same script has a variable defined as: var CurrentMap : MapsClass = null;

On running the game, the variable gets its default value. Then, when I hit the "Lets run" button, the expected level is loaded (another proof that the value was stored), but inside the level, another scripts calls the variable and I get the following error on this line:

 Debug.Log('Player Name: ' + mpManager.PlayerName);            
     if (mpManager.CurrentMap.MapName == 'Londinum' && mpManager.enable_Lap)
     {
         newPlayer.transform.Rotate(Vector3(0,180,0));
     }

 Error: NullReferenceException: Object reference not set to an instance of an object

The Debug line retrieves a correct value from my "Do not destroy on load" super script which is here refered as to "mpManager". The value not recognized is this one: mpManager.CurrentMap.MapName.

Question: Since this thing was running ok in unity3, WTF do I need to do in order to make it run in unity4?

If you help me get the solution a paypal deposit of $20 will be delivered to your account. Thnx in advz for any answer.