iWork '09 retail boxes no longer come with a serial number. Install iWork '09 from the enclosed disc and you're ready to go.
Though retail versions of iWork '09 no longer come with serial numbers, you will need a serial number if you are using the iWork '09 trial version and decide to purchase the fully-functional version of iWork '09.
To upgrade to a fully-functional version of iWork '09, open any of the trial applications. Click the "Buy" button and enter the requested information. An email will be sent to you with the serial number you need to upgrade.