onRelease move movieclip

Hi,

Im making a navigation for a site, and the client wants a box behind a menu item to show that it is the active option. He wants that box to move to behind the new option when selected.

So my navigation goes along the x axis; home contact about etc
initially the box, which I have given the instance name ‘tab’ is behind ‘home’ as its the homepage.

So by clicking on say, the third option, the box will move from home to the third option, and say the user then clicks the second option, the box is to move to behind that.

I got 4 navigation items, all are buttoms, all on their own layer with the box being on its own layer too (obviously below the layers for the buttons so it appears behing the menu item)

I thought it would be quite simple, have some code which effectively says:

buttom1.on(release) {
tab.moveto(x value, 40);
}

but I was wrong. HOW DO I DO THIS???

thanks

Ste