Professional :: Simple Animation Using X And Y Axis

My problem relates to an animation that uses the x and y axis.
for(loop =0; loop < 5; loop++){
boxName = "box" + loop;
attachMovie("box", boxName, 10 * loop);
var randomPlace:Number= random(500) + 550;
var NumX:Number = randomPlace;
[Code] .....

What the code above does is displaying small boxes that are moving from the right to the screen to the left, and then they come back again (like a loop I suppose) and each time a 'box' re-appear, it starts off at a random place at the very top of the screen. I want to make it that it drops just from the top to the screen to the bottom. ( Like snow falling from the sky). But I can't get my ahead around the x / y axis. Also, the size of my frame is 550x400.