Define a symbol already on stage (spectrum analyzer)

Please see the attachment. I am using an opensource spectrum analyzer to try to learn, but I would like to start by making the stage symbol pulse in the same fashion. Thanks,

NOTE that I have inserted the below text at the bottom of the .as file so that you can see how it is not referencing.

//test to see if the MC can be referenced.
_root.mc_ball.scaleX = 100;

I want to control a movie with the instance name mc_ball that I already have on stage.
If I reference it in the .as file via _root.mc_ball.scaleX or whatever I need to do, it says it is not defined…obviously.
So what do I need to do, to define a movie ALREADY created?

I was informed on a different forum that _root is no longer valid, so I am not sure where to go from here. Does anyone know of a tutorial that I could learn about this feature of referencing library/stage symbols? Thanks