Unity RayCast Selection

I've Denoted to simply trying to only select 1 unit.

 var isSelected : Array[];
 var mesh_on : boolean;
 
 isSelected = false;
 mesh_on = false;
 
 function Update () {
 
 var Units = 1 << 8;
 var Ground = 1 << 9;
 
 var ray = camera.ScreenPointToRay(Input.mousePosition);
 var hit_unit: RaycastHit;
 
     if(Input.GetMouseButton(0)){
         var didHit: boolean = (Physics.Raycast(ray, hit_unit, Mathf.Infinity, Units));
         if(didHit){
         print(hit_unit.transform.name);
         isSelected.Add(hit_unit);
         }
         else{
 
         print("you hit nothing");
         }
     }
     if(Input.GetKeyDown("w")){
     hit_unit.transform.Translate(0, 0, 1);
     }
 
 
 var ray_ground = camera.ScreenPointToRay(Input.mousePosition);
 var hit_ground: RaycastHit;
 }

I don't understand why, but I'm getting Cannot convert boolean to Gameobject/Array :\ I'm sure there are other errors, but I'm going slightly insane now not finding away to Raycast and hit then add the hit object to a Selected group.