Actionscript is great! So much easier than other languages. I've been trying to get the enemies to bob up and down for a while, and FINALLY figured it out.
In the EnemyShip class, under var speed; add the line: var bobCounter Then in function onLoad(), add the line bobCounter = 0; Finally, go to function onEnterFrame() and add two lines of code. The first is: _y += Math.cos(bobCounter)*10 and the next line is: bobCounter += (Math.PI/8);