first, insert an event:
<img src="http://dl.getdropbox.com/u/1013446/untitled1.PNG">
then choose the sprite, or whatever object has the timer behavior on it
<img src="http://dl.getdropbox.com/u/1013446/untitled2.PNG">
then make sure to click on the timer tab
and insert the On Timer event
<img src="http://dl.getdropbox.com/u/1013446/untitled3.PNG">
after you tell it the name of your timer, you will have an On Timer event
add any actions to this event that you want when the timer is up
<img src="http://dl.getdropbox.com/u/1013446/untitled4.PNG">