(Sorry, I don't have FC6, so I can't check the new system-config-printer.)
Say you are adding a new printer using the wizard. You get to the screen "Printer model" when you realize you forgot to add the PPD. The wizard doesn't let you add a PPD at this point, and since the wizard is modal, you can't add a PPD from the main window. So, you have to abort the wizard to add a PPD.
It would be easier and intuitive to have an "Add a PPD" button on the Wizard's "Printer model" page.
Andrew
desktop@lists.fedoraproject.org