The normal process for booting the BeOS on a Power Mac involves booting the Mac OS. During the boot process, the Mac OS loads all enabled control panels and extensions, including Be's "OS Chooser" extension. Loading the OS Chooser extension will allow you to select an operating system to load. If you choose the BeOS, the Mac OS load process will stop, and the BeOS will begin to load.
You may have an extensions manager, such as Now StartupManager or Conflict Catcher, which can be set to not load new extensions unless you manually enable them. Check your extensions manager, and make sure the "OS Chooser" extension is turned on, and then reboot.
Be sure to watch your screen during the boot process. The OS Chooser extension will choose a default OS if you do not choose one a few seconds after it shows its dialog box. The first time through, this will be the Mac OS.
Note that you should also have an application called "BeOS Launcher" in a folder called "BeOS Mac Tools." If you launch that application, it will take you directly into the BeOS from the Mac OS without having to restart your Macintosh.