I believe that you coded it yourself but it is some of the easiest code to memorize so not really that impressive.
Try making the movieclip do something else when you press a button (now there is a challenge) and try not using the arrow keys (you do not really have to but it would be better right?)
I will give you some hints:
_width
_height
_alpha
And for other keys:
ASCII
Now get coding (you do not have to but it is great learning new things right?)