There are two types of Prism license activation, and your Prism version and license type will dictate how you activate Prism (with either your Prism serial number or your email address).
This page explains the difference, and steps you through the activation process for both:
http://www.graphpad.com/support/faq/activating-a-prism-license/
And this page explains how to further troubleshoot problems with Prism serial number activation, if needed:
http://graphpad.com/support/faq/troubleshooting-prism-activation/
If this information does not resolve this for you, please reach out to us at help.graphpad.com with your serial number, order information, or any additional information you can provide for further assistance.