If you have erased your hard drive at some point, and reinstalled OS X, then your OS 9/Classic System Folder will be gone. Check your hard drive for a folder named 'System Folder' (and not simply 'System'). If this exists on your hard drive (open the System Folder, and look for various folders (Control Panels, Extensions, Apple Menu Items, lots of others) then you already have OS 9 installed. As a final check, look for Finder and System in that same System Folder. These 2 files are the primary items that make a 'real' System Folder that is usable by your computer (actually the Classic environment will have someplace to call home!)
If this items can't be found, then do find one of the CDs that came with your PowerBook, the one called Applications (or something similar). Run the installer on that disk. This will give you a choice to install a variety of apps. One will be OS 9 Classic. Do that. And that's it. (oh yeah, run the Classic pref pane and select your System Folder, and start Classic so your system will update Classic with needed pieces from your System.
Last note: Just because OS 9 is installed, does not mean you can boot to OS 9. You can't. Your PowerBook can't boot from OS 9, and can't boot from an OS 9 install CD. But, you can run older software from the Classic OS 9 environment.