trying to move an object with the mouse

hey everyone,

i'm trying to move an object with a mouse and give it depth i've tried to add force and velocity and seem to be giving it either no movement or way way to much and shooting it off the screen.

i want to move the object towards the center of the screen when moving it around with the mouse, here is the code i have attempted so far but still having issues with.

 var dragSpeed=1.0;    
 private var mag: float;;  
 public var isDragging : boolean=false;
 static var dragging : boolean = false; // for camera use
 private var obj : GameObject;
 
 function Awake(){    
     obj = GameObject.Find("stone_point");    
     mag = Vector3.Magnitude(Vector3((obj.transform.position.x - Camera.main.transform.position.x), 0, (obj.transform.position.z - Camera.main.transform.position.z)));    
     }
 
     function OnMouseDown(){    
         var ray : Ray = Camera.main.ScreenPointToRay(Input.mousePosition);    
         rigidbody.useGravity = false;    
         rigidbody.freezeRotation = true;
         rigidbody.AddForce((transform.position - ray.GetPoint(mag))*8.0);    
         //rigidbody.velocity=(transform.position - ray.GetPoint(mag));
         dragging = true;
         } 
 
     function OnMouseUp(){    
     rigidbody.useGravity = true;    
     rigidbody.freezeRotation = false;    
     dragging = false;    
     }

thanks for any and all help!