Flash player 15 ActiveX is Flash player 15 plugin..
Do you mean stand alone flash player(exe)?
or Do you mean Flash Player plugin for FireFox?
In any case, this is Adobe Media server forum...Please post this question on Flash Player forum.
You can download the latest version of Flash Player by going to http://get.adobe.com/flashplayer. Various browsers need different versions of Flash Player. Installing Flash Player on Internet Explorer will not make Firefox work, for instance. If you're running into this kind of situation, just visit the link again using the browser that isn't detecting Flash Player.
If you're still having trouble after that, some details about your system config would help us give you a more precise answer: