The class or interface of MouseEvent could not be loaded

1

Hi! I am new in flash. I am using Flash CS4 - AS2.

I am currently doing this gallery with prev and next button. But i got this error message…
“The class or interface of MouseEvent could not be loaded.” It says that i am having a problem with the functions that I have.

stop();

btn1.addEventListener(MouseEvent.CLICK,backward);
btn2.addEventListener(MouseEvent.CLICK,forward);

function forward(event:MouseEvent){
if(this.currentFrame == this.totalFrames){
gotoAndStop(1);
}
else{
nextFrame();
}
}

function backward(event:MouseEvent){
if(this.currentFrame == 1){
gotoAndStop(this.totalFrames);
}
else{
prevFrame();
}
}

How can I resolve this? thanks!