Draw a circle (or whatever shape you want), select it and hit F8 to convert
it to a movieclip, name this 'ball'.

Actionscript Breakdown

A break down of how the random motion script will work

1. Generate a random value for the position
   
2. Move the object gradually towards the random position value
   
3. Once it reaches the position generate another random value, repeat the
   process

Note: All the actionscript used in the tutorial will be added on frame one of
the movie clips timeline and not the main timeline (for simplicity's sake
)

1. Generating the Random Position

First we need to generate a Random Position, we can generate a position
pretty easily by using

newpos = function () {
ranx = Math.random ()*250)
rany = Math.random ()*250)
}

this.onEnterFrame = function() {
    newpos ()    
    this._x = ranx;
    this._y = rany;
}

In the function newpos () Math.Random () generates a random decimal number
between 0 and 1, it is then multiplied by 250 which is the width of the movie.
So in effect ranx and rany the random positions will be set to value

The this.onEnterFrame part calls the function newpos()
continuously and sets the position of the movieclip to a random value

2. Gradually moving towards the random position

acceleration = 10
newpos = function () {
 ranx = Math.round((Math.random ()*250));
 rany = Math.round ((Math.random ()*250));
}
newpos();
this.onEnterFrame = function() {
 this._x += ((ranx-this._x)/ acceleration);
 this._y += ((rany-this._y)/ acceleration);
};

newpos() has been moved outside the onEnterFrame event because we
want to move the movieclips gradually not every time the movieclip enters a
frame.

this._y += ((rany-this._y)/ acceleration);

this._y = ((rany-this._y)/ acceleration) + this._y;

The code keeps adding a portion of the difference between the current movie
clip position and the randomly generated position, on enterframe.

This way the the _x and _y values of the movieclip are gradually increased to
match the the values of the randomly generated position.

3. Repeating the process

The process needs keep repeating so we add the following code.

acceleration = 10
newpos = function () {
 ranx = Math.round((Math.random ()*250));
 rany = Math.round ((Math.random ()*250));
}
newpos();
this.onEnterFrame = function() {
 this._x += ((ranx-this._x)/acceleration);
 this._y += ((rany-this._y)/acceleration);
 if (Math.round(this._x) == ranx || Math.round(this._y) == rany) {
  newpos();
 }
};

This way once the _x value of the movieclip equals the random position. The
newpos() function is called again to generate a new set of random x and y values
and the whole process repeats itself over and over again.

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Source