MonoDevelop doesn't recognize Priority_Queue library

I'm following a tutorial of a guy who is using all from scratch, i mean, monodevelop from unity and unity3d just downloaded, no plugins, no nothing else and in the free version.

The Tutorial use the up-to-date version (28-02-2016) of unity and as a consecuence of that of the MonoDevelop included in unity, is in fact taking new episodes as we speak.

The thing is, by some odd reason, my MonoDevelop doesn't recognize Priority_Queue library, and it seems to happend just to me, none of the other guys out there have this problem aparently.

I try re-downloadeing Unity3d, and reinstalling it, doesn't work.

So, the question would be, could somebody tell me how to fix that problem?

Any request for clarification, comment, or constructive question than or answer the question or improve the same, would be much apreciated.

Here is some code added just to know what could possibe be wrong,

 using UnityEngine;
 using System.Collections.Generic;
 using Priority_Queue;
 
 public class Path_AStar {
 
     Queue<Tile> path;
 
     //Tile nextTile;
 
     World world = WorldController.Instance.world;
 
     public Path_AStar(World world, Tile tileStart, Tile tileEnd){
 
         // Check to see if we have valid tile graph.
         if (world.tileGraph == null) {
             world.tileGraph = new Path_TileGraph ();
         }
 
         // A Dictionary of all valid, walkable nodes.
         Dictionary<Tile, Path_Node<Tile>> nodes = world.tileGraph.nodes;
 
         Path_Node<Tile> start = nodes [tileStart];
         Path_Node<Tile> goal = nodes [tileEnd];
 
         // Make sure our start/end tiles are in the list of nodes!
         if (nodes.ContainsKey (tileStart) == false) {
             Debug.LogError ("Path_AStar: the starting tile isn't in the list of nodes");
             return;
         }
         if (nodes.ContainsKey (tileEnd) == false) {
             Debug.LogError ("Path_AStar: the closing tile isn't in the list of nodes");
             return;
         }
 
         // Mostly following this pseudocode:
         // http://en.wikipedia.org/wiki/A* _search_algorithm
 
         List<Path_Node<Tile>> ClosedSet = new List<Path_Node<Tile>> ();
 
     //    List<Path_Node<Tile>> OpenSet = new List<Path_Node<Tile>> ();
     //    OpenSet.Add (start);
 
         SimplePriorityQueue<Path_Node<Tile>> OpenSet = new SimplePriorityQueue<Path_Node<Tile>> ();
         OpenSet.Enqueue ( start, 0);
 
         Dictionary<Path_Node<Tile>, Path_Node<Tile>> Came_From = new Dictionary<Path_Node<Tile>, Path_Node<Tile>> ();

The code continue after that, but it doesn't seem to matter, if is not recognizing the code i can just do very little

Thanks in advance.