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