The email that contains the activation password is generated within a few seconds of when you submit the registration form. There are several reasons why the email might not arrive, such as a error in your email address, problems with your email server, delays across the Internet, spam filters, etc.
Spam filters are the most likely source of a problem. The 30-day registration passwords are sent from "info@respondus.com" so it may be necessary to add this to your "address book," "safe sender," or "buddy list." If an entire domain can be specified, enter "respondus.com"
If the email doesn't arrive within 15 minutes, try submitting the online registration form again. You might also try using a different email address. (Note: Free email services are known for delayed emails and service outages.)