See these links and the related knowledge base articles listed below for answers to some common questions:
Authenticating with Duo - Getting started and logging in with Duo, changing your settings, and adding a new device.
Enrolling with Duo - Enrolling your phone or tablet for Duo 2FA.
If you already use Duo and find yourself locked out, please try again after 10 minutes or call 303-497-2400 for assistance.