Installer - Action: Disabled

[motulist]

I'm trying to downgrade and install an older version of itunes (4.8) and when I launch the installer and go through a few screens I get to the choose Installation type, but all the choices are greyed out and under the action column it says "Disabled". How do I get it to let me install?

pb 1 ghz
10.3.9

p.s. you can read about why I want to downgrade here http://forums.macrumors.com/showthread.php?t=141765