Blindly getting the names of movieclips on root


I have been struggling with the following scenario.

I have two movieclips on stage.

mcPlayMovie and mcMainClip

I use the getURL to dynamically load a movieclip (childMovie.swf) into mcPlayMovie.

I am trying to access mcMainClip from this embedded mc in mcPlayMovie.

from the as code in childMovie.swf, I try to call this.root.mcMainClip.SetValue();

However, it says that mcMainClip is not valid.

So how can I access this mc sitting on the stage from an embedded mc?

I am using CS4, AS3.

Thank you all!