ActionScript 3.0 :: Creating Custom Button Class Into Main Timeline

I had watched a CartoonSmart Tutorial on AS3, and the amount of code that goes into a button seems to much (especially coming from AS2). I tried to create a custom class to make my life easier for this, but... I ran into a snag.

Errors:
Code:
1046: Type was not found or was not a compile-time constant: MouseEvent.
I receive that error 4 times in these locations.

Code:
function ButtonClick(event:MouseEvent):void {
function ButtonRollOver(event:MouseEvent):void {
function ButtonRollOut(event:MouseEvent):void {
function ButtonDown(event:MouseEvent):void {

Here is my button_class.as file.
package {
public class button_class {
function button() {
}buttonVar.addEventListener(MouseEvent.MOUSE_UP , ButtonClick );
function ButtonClick(event:MouseEvent):void {

I am using Flash CS3 Professional. And when I have the class put directly into the main time line ( Copy and paste) It works perfectly, but not here.