Displayind a centered pop-up window (for Kirupa?)

Displayind a centered pop-up window
Hello, I was trying to use kirupa’s code for displayind a centered pop-up window when a flash button is hitted. I have followed the indications posted on http://www.kirupa.com/developer/mx/centered_popup.htm but so far it does not work in my try out. My button is a movie clip on the up state of the button, that loops and never stops. Is that a problem? Another question is that I interpreted that I must change _root.openWinCentre What shall I write instead of “_root”? In his sample he writes: Movieclip.prototype.openWinCentre But I cannot find in his button any reference to “Movieclip.prototype”. I have copied the code without any change and does not work either.
Maybe the code does not work when made in Flash MX 2004? (The original code was made with Flash MX)

Any help, would be very very welcome!! Thanks you!