Hi, It should show up in your start menu. It will create an entry called "WCM Live Broadcaster". Did you download it from the Broadcaster page? (
http://www.warcraftmovies.com/broadcaster.php)
Did you install it afterwords? After installing the software it will end up in the System Tray. Right Click its icon and select settings to enter the settings. To start a Broadcast hit F7 (default).
Please note that the Plugin in Firefox is only for viewing a stream. Not broadcasting your own.