• Define variables.

• Generate random numbers for the x and y positions.

• Gradually move the object to this position.

• Tell object to to repeat the process when it reaches the new position.

accel = 4;
var randx = Math.round(Math.random()*Stage.width);

ball_mc.onEnterFrame = function (){
ball_mc._x += (randx-ball_mc._x)/accel;

if(Math.round(ball_mc._x)==randx){
randx = Math.round(Math.random()*Stage.width);
}
}