Movement works in mx,not mx 04

hey, I use flash mx, but i’ve been staying at a friends house lately who uses mx 04, i was wondering a few things like…how come this movement script works in mx, but not mx 04…
placed on a mc that i want to move around

onClipEvent(enterFrame) {
if(key.isDown(key.RIGHT)) {
this._x += 5;
else if(key.isDown(key.LEFT)) {
this._x -=5;


so why does this work for mx, but not mx 04