New User - "Challenge" links don't seem to be active

Hi - Hoping someone might be able to help out. I just set up the squash ladder on the hosted server (http://squash-ladder.net/UCSF), and it looks great! Thanks so much for developing this!

One question, though - When I go to the "rankings" page, I see a list of registered members, but there is no way to click on their name to extend a challenge. Am I doing something wrong? Is there something a member has to configure in order to be "challenge-able?" I know this is really basic, and I apologize, but I really need to know how to do this, as people are beginning to sign up!

Thanks so much for any help you can provide.