This will be short and simple. Make a layer called actions. Type in it:
createEmptyMovieClip("terrain",1);
terrain.lineStyle(10,0x000000,100);
imdrawing = false;
onMouseDown = function () {
 if (imdrawing == false) {
  terrain.moveTo(_xmouse,_ymouse);
  imdrawing = true;
 }
 if (imdrawing == true) {
  onEnterFrame = function () {
   terrain.lineTo(_xmouse,_ymouse);
  };
 }
};
onMouseUp = function () {
 onEnterFrame = function () {
  imdrawing = false;
 };
};
That just create a line and said when I click start drawing, and when I release stop drawing. If you want to change the color,width, or alpha heres what to do. To change the width, just type a number where the code is Light Blue.A small number is a small width, a big number a big width. To change the color take a colors Hex number and enter it where the code is Orange. Simple ya. The alpha is where the code is green, type a number 1-100 right there.And there you have it!Leave a comment if you need help!