Help with movement animation

Please help me make it so when my character moves is plays the whole movement animation and not just one frame. Heres the code:

onClipEvent (load) {
moveSpeed = 4;
onClipEvent (enterFrame) {
if (Key.isDown (Key.RIGHT)) {
this._x += moveSpeed; _xscale = -100;
if (Key.isDown (Key.LEFT)) {
this._x -= moveSpeed; _xscale = 100;

the movemnt animation is 10 frames long but it only plays the first frame in the movie clip and then stops.

I want it to be something like this

Please help me Chen!