This looks like old AS2 loadclip code. Things like _yscale only exist in ActionScript 2.0 not 3.0. Is this a 3.0 project? If so it would be rectMask_mc.scaleY.
It was very common in AS2 to have a SWF contain its own preloader. In AS3, a smaller outer clip (usually called a stub) is loaded with enough code to get the larger SWF preloading while displaying progress.
Are you trying to preload a whole single CS6 project you did or have your project preload another clip into itself?