Are you getting a message that says to enter the serial number or to buy one even though you've already bought the program? If so, this is because you have the trial of iWork on your Mac. 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.
Yvan Koenig has written an 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.