Keeping the selected menu active on the navigation bar

I’m a newbie to action script 3.0 and would like some help with the menu bar. I have created a website in Flash CS5 with 5 buttons on the menu bar, each goes to a different page. Now how do I keep the down state of the button when that page is clicked? so the animation plays till the user stays in that page?

This is my first post and hope I can find someone who can help. Thank you so very much!