I would recommend to create your component in a separate mxml file or even better in a pure AS3 based component. To do what you want to do, you could create a handler function in your Script area for that click event, and in there assign the instance of the component and then call any method before calling open, like:
click = "clickHandler()"
----- inside your script
function clickHandler : void {
var a : AlertMsg = new AlertMsg();
a.setMetaDataXML("foo");
a.open(this,false);
}
I would also recommend you to declare implicit setters and getters the AS3 way