Why error message?

Question

I am attempting to learn to do in Flash some things I have
typically done in Director. Needless to say, it's been a tough go.
I have been using some of the Lynda.com stuff and it's been
helpful, but I now have a problem I can't figure out. I am using a
script that Lynda.com uses in their tutorial movies. I copied it
(except using my names for things, of course) and from what I can
tell, I did it correctly, but I keep getting an error message. Here
is my action script (in it's own layer):
stop();
function onIntroClick(evt:MouseEvent):void {
gotoAndStop("intro");
introbut2jpg.addEventListener(MouseEvent.CLICK,
onIntroClick);
It's just a graphic I turned into a button that should jump
to the frame labeled "intro" and stop there. Works great in the
tutorial, but I get:
Location: Scene = Scene 1, layer = Actions, frame=39, Line 3
Description: The class or interface 'MouseEvent' could not be
loaded.
Source: function onIntroClick(evt:MouseEvent):void {

Answer 1

Here is the code for all 3 buttons playing the flvs:
stop();
var tl:MovieClip = this;
function onIntroClick(evt:MouseEvent):void {
tl.gotoAndStop("intro");
intro_butjpg.addEventListener(MouseEvent.CLICK,
onIntroClick);
stop();
function onTeachingClick(evt:MouseEvent):void {
tl.gotoAndStop("teaching");
teachbut.addEventListener(MouseEvent.CLICK, onTeachingClick);
stop();
function onMentalClick(evt:MouseEvent):void {
tl.gotoAndStop("mental");
mentalbut.addEventListener(MouseEvent.CLICK, onMentalClick);
I don't understand why it works with 1, then with 2, but not
with 3.