Actually, there is nothing missing from your iMac box. The registration notice that comes up when you run ClarisWorks is one of the items left behind from its previous development at Claris Corporation and will be removed from Apple's rebranded version, AppleWorks.
The one registration card that came with your iMac is sufficient to register all Apple products included with your computer, including ClarisWorks. Please take a moment to fill it out and mail it in, or follow the instructions on the card to register over the World Wide Web.
To avoid any further inconvenience, it is recommended that you click the Never button in the ClarisWorks registration notice. The registration card produced by this notice has outdated return address information and should not be used as a substitute for the card that came in your iMac box.