There's a good example of this at:-
http://rhuno.com/flashblog/2011/11/18/calculating-angles-and-moving-objects-accordingly/
You want to do something like
var angle:int = 270;
var rads:Number = angle*Math.PI/180;
var speed:int = 2;
function moveBullet(e:Event)
{
e.currentTarget.x += Math.cos(rads) * speed;
e.currentTarget.y += Math.sin(rads) * speed;
}
You'll need to set angle accordingly depending on which way the character is facing (if it flips)