I am not quite sure what the problem is as I am a beginner as well and still learning, but with my experience in VB.net, it looks to me like you have a function nested within a function.
I'm not sure if that is possible in AS 2.0, but that might be the problem.
Exactly as said as above. For clarification it is good to note that button instances have different methods and functions you can use from movieClips. For example, if Start_btn is on the instance of a button, you can not use onEnterFrame -because that method is only for movieClips.
So yes, use Start_btn.onRelease instead. But other than that your organization and code layout looks well done, nice!