Note that Apple's registration servers for iWork '06 have been offline for years. While you can still enter a valid serial number to unlock the local software, you won't be able to "register" it with Apple online. Apple Support Community
: A common "bug" occurred when users tried to use a retail serial number on a pre-installed trial version. The trial was often a slightly different sub-version (e.g., v1.0.1 vs v1.0.0), causing the serial to be rejected. The solution was to delete the trial version and its associated preference files before installing from the retail disc. Extracting a Lost Key iwork 06 serial number