The class or interface 'MouseEvent' could not be loaded

Question

I am creating a swf file that would get populated with images
and text taken entirely from an XML file from the local folder. All
i need to do now is manouver between the images using a click of a
button. Say there are some images lined up one below the other and
there would be a button on screen.. on click of which the focus has
to shift from one image to the other. To achieve this functionality
i have created a Movie Clip that would act as the BG for the Images
to show that the image is in Focus. I check the current location of
the Movie Clip on cick of the button and then set the next location
of the Bg movie clip accordingly.
The Functionality of moving/changing coordinates of the movie
clip on click of a button works fine when done separately in a new
file, but when combined with the code where the XML gets parsed and
the Imges are set on the UI, it gives me an error -- "The class or
interface 'MouseEvent' could not be loaded"
I'm adding my code below
RPNext_btn.addEventListener(MouseEvent.CLICK, clickHandler);
function clickHandler(event:MouseEvent):void {
trace("here");
var XCo = SwatchBGRght_MC.x;
var YCo = SwatchBGRght_MC.y;
trace("X coordinate::"+XCo);
trace("Y coordinate::"+YCo);
if(YCo==145){
//SwatchBGRght_MC.y = 282.2;
if(YCo==282.2){
//SwatchBGRght_MC.y = 419.4;
if(YCo==419.4){
//SwatchBGRght_MC.y = 145;
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Answer 1

Well, I guess I'll have to see the code, that is NOT
functioning, to track this error down.