straight from the help docs
var checkboxListener:Object = new Object();
checkboxListener.click = function(evt_obj:Object)
{
if (evt_obj.target.selected)
{
evt_obj.target.label = "Selected!";
trace("checkbox is selected");
} else
{
evt_obj.target.label = "Unselected!";
trace("checkbox is deselected");
}
};
my_ch.addEventListener("click",checkboxListener);