Unity instantiate object in a grid?

I'm trying to get some objects to spawn in Unity on the a grid which I located. However while the objects do spawn on click, they are not placed on the grid. The objects spawn in a random location off the actual playing field.

I've inserted the script used.

Code (JavaScript):

1. //NGU items
2.  
3. //var buildPanelOpen : boolean = false;
4. //var buildPanelTweener : TweenPosition;
5.  
6. //Placement Plane Items
7. var placementPlanesRoot : Transform; // attach the placement plane root
8. var hoverMat : Material; // attach the hover material to this slot
9. var placementLayerMask : LayerMask; // calls up the layer mask selection
10.  
11. private var originalMat : Material;
12. private var lastHitObj : GameObject;
13.  
14. //Build Selection Items
15. var onColor : Color;
16. var offColor : Color;
17. var allStructures : GameObject[];
18. //var buildBtnGraphics : UISlicedSprite[];
19.  
20. private var structureIndex : int = 0;
21.  
22. function Start ()
23. {
24.     //reset the structure index, refresh the GUI
25.     structureIndex = 0;
26.     UpdateGUI();
27. }
28.  
29. function Update ()
30. {
31.  
32.         var ray = Camera.main.ScreenPointToRay ( Input.mousePosition );
33.         var hit : RaycastHit;
34.  
35.                 if ( Physics.Raycast ( ray, hit, 1000, placementLayerMask ) )
36.                 {
37.                     if ( lastHitObj )
38.                     {
39.                     lastHitObj.renderer.material = originalMat;
40.                     }
41.                 lastHitObj = hit.collider.gameObject;
42.                 originalMat = lastHitObj.renderer.material;
43.                 lastHitObj.renderer.material = hoverMat; // sets the planes material to the highlighted material
44.                 }
45.  
46. else // if the raycast didn’t hit anything
47.         {
48.             if ( lastHitObj ) // if we had previously hit something
49.             {
50.             lastHitObj.renderer.material = originalMat; // visually de select that object
51.             lastHitObj = null; // nullify the plane selection
52.             }
53.         }
54.  
55.         // drops a turrent on click
56. if ( Input.GetMouseButtonDown ( 0 ) && lastHitObj ) //left mouse button was clicked and there is a last hitObject // if left mouse button (0) is clicked and we have something selected
57. {
58.     if ( lastHitObj.tag == "Placement_Open" )
59.     {
60. // drop the chosen structure exactly at the tile’s position and rotation of Zero. See how the “index” comes into play here?
61.     var newStructure : GameObject = Instantiate( allStructures[structureIndex], lastHitObj.transform.position, Quaternion.identity );
62. // set the new structure to have a random rotation. just for looks
63.     newStructure.transform.localEulerAngles.y = ( Random.Range ( 0, 360 ) );
64. // set this tile’s tag to “Taken” so we can’t double place a structure
65.     lastHitObj.tag = "Placement_Taken";
66.         }
67.     }
68. }
69.  
70.  
71. function UpdateGUI ()
72. {
73. /*//Go through all structure buttons (buttons in the build panel). and set them to “off”
74. for ( var theBtnGraphic : UISlicedSprite in buildBtnGraphics )
75. {
76. theBtnGraphic.color = offColor;
77. }
78. //set the selected build button to “on”
79. buildBtnGraphics [ structureIndex ].color = onColor;
80. */
81. }
82.  
83.  
84. // Called whenever a structure choice is clicked ( the button in the build panel )
85. function SetBuildChoice ( btnObj : GameObject )
86. {
87. var btnName : String = btnObj.name;
88.  
89. if ( btnName == "Button_Turret" )
90. {
91. structureIndex = 0;
92. }
93. else if ( btnName == "Button_Mine" )
94. {
95. structureIndex = 1;
96. }
97. else if ( btnName == "Button_Sam" )
98. {
99. structureIndex = 2;
100. }
101. UpdateGUI ();
102. }

Would appreciate any help I can get on where I might have gone wrong.