Calculating speed with rigidbody.velocity.magnitude

Okay, I know that you can calculate speed with rigidbody.velocity.magnitude;, but how to do that? Can someone of you, guys, show me an example. Like this: var SpeedTXT: GUIText;

 function Update(){
 var rawSpeed = rigidbody.velocity.magnitude;
       Time.timeScale = 1;
       SpeedTXT.text = "Speed = " + rawSpeed;
 }

Actually that was my script, and the problem is that it shows me only 0(even when my plane moves). So what should I do? Please help.