Use random numbers. Let's say you wanted the enemy's x position to be between 0 and 500. You use the function
Math.random();
which will return a number between 0 and 1. Multiply that number by 500 to get a number between 0 and 500, then assign it to your enemy
var randomX:Number = Math.random()*500;
enemy.x=randomX;
You would need to do it for y as well. if you don't want the enemy to appear off the screen you need to adjust the formula to take the enemy's height and width into account
Alternatively if you wanted the enemy to appear in set positions (IE, top left bottom or right), then store those points in an array
var startPositions:Array = new Array();
startPositions.push(new Point(10,10));
startPositions.push(new Point(400,10));
startPositions.push(new Point(10,400));
startPositions.push(new Point(400,400));
then pick out a random spot in the array to use
var randomStartIndex:uint = Math.floor(Math.random()*startPositions.length);
var randomPoint:Point = startPositions[randomStartIndex];
enemy.x = randomPoint.x;
enemy.y = randomPoint.y;
this code is in AS3, if you're using as2 it will be slightly different (mostly just the use of _x and _y instead of x and y);