The retail OS X install discs don't install OS 9. And 10.5 does not support the classic environment. If you have 10.5 installed, OS 9 can exist on another partition or disc and be booted. The only way to install OS 9 on a MDD is from the restore discs that came with the computer, or copy the system folder from another computer that has a working install of 9.2.2. I sent you a link that should help.