Instead of adding a constant 10, you can have a variable, and change that variable value.

You should also consider keeping int variables for all your numbers rather than using the text values in the textfields.
Code:
var dropSpeed:Number = 10;
var atrapadasNum:int = 0;
var erradasNum:int = 0;

function crearID():void{
	var enemigo:Cayendo=new Cayendo()
	enemigo.y=-50;
	enemigo.x=Math.random()*stage.stageWidth;
	enemigo.addEventListener(Event.ENTER_FRAME,dropEnemy);
	addChild(enemigo);
}

function dropEnemy(e:Event):void{
	var mc:Cayendo=Cayendo(e.target);
	mc.y+=dropSpeed; 
	if(mc.hitTestObject(agarrador)){
		atrapadas(mc);
	}else if(mc.y>stage.stageHeight){
		erradas(mc);
	}
}
function atrapadas(mc:Cayendo):void{
	mc.removeEventListener(Event.ENTER_FRAME,dropEnemy);
	removeChild(mc);
        atrapadasNum++;
	atrapadasText.text=String(atrapadasNum);
	if(atrapadasNum == 50){
                dropSpeed = 60;
		nivel2.gotoAndPlay(1);
	}
}
function erradas(mc:Cayendo):void{
	mc.removeEventListener(Event.ENTER_FRAME,dropEnemy);
	removeChild(mc);
	erradasNum++;
	erradasText.text=String(erradasNum);
	if(erradasNum == 5){
		gameOver();
	}
}

function gameOver():void{
	score=atrapadasNum;
	removeChild(agarrador);
	clearInterval(crearIDenemigo);
	removeChild(atrapadasText);
	removeChild(erradasText);
	while(numChildren>0){
	  getChildAt(0).removeEventListener(Event.ENTER_FRAME,dropEnemy);
  	  removeChildAt(0);
	}
	stage.removeEventListener(KeyboardEvent.KEY_DOWN, presionoTecla);
	stage.removeEventListener(KeyboardEvent.KEY_UP, sueltoTecla);
	stage.removeEventListener(Event.ENTER_FRAME , mover);
	gotoAndStop("game over");
	Mouse.show();
}
iniciarJuego();
You should also consider using a Timer rather than setInterval/clearInterval.