You're getting the request for a serial number that means that at some time you had the iWork ’09 trial installed. You need to delete the trial & then reinstall from the boxed DVD or the Mac App Store. The files to delete are the iWork ’09 folder from the main HD > Applications; the iWork ’09 folder in HD > Library > Application Support & the individual iWork application plist files found in HD > Users > (your account) > Library > Preferences for each user. The easist way to fix the problem is to use Yvan Koenig's AppleScript that removes the files. You can find it on his box.com account in for_iWork'09 > other_iWork'09 items > uninstall iWork '09.zip.
Apple's support article on the subject says it's due to having a copy of the trial that is a higher version that the one you've purchased, but I'm not sure that is very common. This problem started with Snow Leopard & it seems to be some code in the iWork installers that doesn't do what it should.