Make your enemy a movie clip, and set the actions for the movie clip as the following:
onClipEvent(enterFrame) {
randomX = Math.random() * (highX - lowX)) + lowX;
randomY = Math.random() * (highY - lowY)) + lowY;
this._x = randomX
this._y = randomY
}
Replace "highX" with the x-value of the farthest right you'd want an enemy appear. Likewise, replace "lowX" with the x-value of the farthest to the left you'd want an enemy appear. Do the same with "highY" (lowest on the stage you'd want enemy to appear) and "lowY" (highest on the stage you'd want enemy to appear.) Hope this works, I'm not 100% sure this is correct.