Timer and gui text

You must create a GUIText and set a reference to it in your script, which will become something like this:

 var gText: GUIText; // drag here the GUIText from Hierarchy view
 var timer: float = 10;
 
 function Update(){
   timer -= Time.deltaTime;
   if (timer < 0) timer = 0; // clamp the timer to zero
   var seconds: int = timer % 60; // calculate the seconds
   var minutes: int = timer / 60; // calculate the minutes
   gText.text = minutes + ":" + seconds;
 }

This formatting doesn't work fine with negative numbers, thus the timer is clamped to zero. If you need to display negative values, change the function Update to this:

 ...
 function Update(){
   timer -= Time.deltaTime;
   var t = Mathf.Abs(timer); // get the absolute timer value
   var seconds: int = t % 60; // calculate the seconds
   var minutes: int = t / 60; // calculate the minutes
   var minSec = minutes + ":" + seconds; // create the formatted string
   if (timer < 0) minSec = "-" + minSec; // add a minus sign if negative
   gText.text = minSec; // update the GUIText
 }

EDITED:

String concatenation isn't a good idea because it generates lots of intermediate memory allocations, which may cause performance hiccups when garbage collection (free memory recuperation) takes place. A better solution is to use .NET/Mono String.Format:

   gText.text = String.Format("{0:00}:{1:00}", minutes, seconds);

Each number is defined in the formatting string inside curly brackets, where the digit before the colon tells which argument to use (starting at 0), and the digits after the colon show the format. Take a look at .NET docs for more details.