actually, it's not really simple per say, but it's doable. About 4 years ago, I was trying to do the same thing and I gave up, but I have since created a technique that allows you to do it. The easiest thing would be for you to just get the sourcecode of something that I've made, but I'll explain how it works.
You have you menu button, and then the submenu pops out of it in some way, right? well the submenu buttons aren't set as "buttons" in the flash program, but just movie clips, then you code the submenu buttons to animate/activate while the mouse is over the submenu "buttons" or movieclips. you write a script to find out where the mouse is in relation to the actual button, because flash doesn't understand buttons within buttons, you can be over one, but not multiple, so you have to hand-code the submenu items as if they were the same as the original button, not a separate button, but a function that's written within the encompassing button. did I lose you? maybe I should write a tutorial, because now that I think about it, I don't see too many people making menus with flash as pulldown menus the way that I do.
Now that you know it's a long answer, if you still want to understand it, I'll send you some sourcecode that I wrote. just let me know.