Firstly, I recommend you start learning AS 3 instead of AS 2, but if your gonna stick with AS 2 then...
This should help you abit. Its badly written and doesnt contain much but you might pick some things up
AS: Random Enemies
Since your a beginner Id recommend doing it this way, even though its not how Id tackle it myself
You need something like if (random(50) == 1) { to get you started. Inside there youd want to have a duplicateMovieClip to copy your enemies.
Then id have a variable 'position' that is a random number up to 4. Each number corresponding to a different side of the screen. At the top, _y = -10 and _x = random(550). At the right side _y = random (400) and _x = 560
This should cover getting your enemy to move around
Hope this helps in some way.