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.
Note: If you are using the trial version of iWork '09 and then purchase a retail box, you will not need a serial number. Install iWork from the disc that came with your retail box to begin using your fully-functional version of iWork '09.