You could use a button with a keyPress event,
Code:
on (keyPress "a") {
	trace("a was pressed");
}
or have a listener object to detect keypresses,

Code:
var kl = {};
kl.onKeyDown = function() {
    if (Key.getCode() == 65) {
        trace("a was pressed");
    } else if (Key.getCode() == 68) {
        trace("d was pressed");
    }
};
Key.addListener(kl);