Display List Issues

I'm new to action script 3.0 programming and I'm learning about the Display List and running into compile errors and I can't figure out what I'm doing wrong. I continue to get the following errors:

1061: Call to a possibly undefined method getChildAt through a reference with static type flash.displayisplayObject.

1119: Access of possibly undefined property numChildren through a reference with static type flash.displayisplayObject.

I downloaded this source code and it worked fine with the example file I downloaded, but when I created a new fla document and used the same code I get the errors above. Can any one point me in the right direction?