Hey Kirupa,
One of the members noticed in the centered pop up tutorial that using Movieclip.prototype doesn’t work for Flash player 7 and 8, though it works fine for earlier versions. Seems it works if one uses MovieClip.prototype (capital C).
Here is the thread:
http://www.kirupa.com/forum/showthread.php?t=205566
Just thought I’d let you know. 
