Class Scope Variable

This code is part of the package Tile Based Map and Nav. What I'm trying to get out of it is, get the node that was clicked on and send an event to playmaker to activate some GUI buttons. When you click one of the buttons it selects the prefab and sends a message from playmaker to invoke the spawn function at the node location. The problem I'm running in to is that when I put the Unit.SpawnUnit line outside of the Update method, it can't find the variable "node".

I've been told that this is because the variable isn't being saved and that I should create a Class Scope Variable to save the node information in. Well, I looked up how to create one and I'm not really understanding it. I need some insight.

 using UnityEngine;
 using System.Collections;
 using HutongGames.PlayMaker;
 
 public class GameControllerIOW : MonoBehaviour 
 {
     // This is a simple sample of how to spawn a unit on a tile that was clicked
     
     public CameraMove camMover; // used to move camera around (like make it follow a transform)
     public Camera rayCam;
     public GameObject unitFab; // unit prefab
     public MapNav map; // the mapnav
     public LayerMask tilesLayer; // layer the tiles are on
     public PlayMakerFSM behavior; // playmaker behavior
     public int nodeId;
     public Vector3 nodeLoc;
     public TileNode node = null;
  
     IEnumerator Start()
     {
         // wait for a frame for everything else to start and then enable the colliders for the TielNodes
         yield return null;
         
         // now enable the colliders of the TileNodes.
         // they are disabled by default, but for this sample to work I need the player to be able to click on any tile.
         // for your game you will have to decide when the best time would be to this or even which tiles would be
         // best to enable. For example, you might only want to spawn new units around some building, so only
         // enable the the tiles around the building so that the player cannot click on other tiles and disable 
         // the tiles whne yo uare done with them
 
         foreach (TileNode n in map.nodes)
         {
             n.collider.enabled = true;
         }
     }
 
     void Update()
     {
         // don;t do anything else if there was not a mouse click
         if (!Input.GetMouseButtonUp(0)) return;
         
         // cast a ray to check what the player "clicked" on. Only want to know 
         // about TILE clicks, so pass mask to check against layer for tiles only
         Ray ray = rayCam.ScreenPointToRay(Input.mousePosition);
         RaycastHit hit;
         if (Physics.Raycast(ray, out hit, 500f, tilesLayer))
         {
             // a tile gameobject was clicked on
             
             // get the TileNode
             TileNode node = hit.collider.GetComponent<TileNode>();
             if (node == null) return; // sanity check
             
             nodeId = node.idx;
             //nodeLoc = node.transform.position; 
             
             // dont spawn here if there is alrelady a unit on this tile
             //if (node.units.Count > 0) return;
             
             // send events to Playmaker
             behavior.Fsm.Event("TileNodeActive");
             
             //Unit.SpawnUnit(unitFab, map, node);
         }
     }
     void Spawn ()
     {
         if (node == null) return; // sanity check
         
         // dont spawn here if there is alrelady a unit on this tile
         if (node.units.Count > 0) return;
         
         Unit.SpawnUnit(unitFab, map, node);
         
         behavior.Fsm.Event("Unit Spawned");
         nodeLoc = node.transform.position;
     }
 }