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You have to put either their nickname or the email address they use at PBS in the box. You can't just put their name. (Funny story, I got a referral credit from somebody I'd never heard of one time, because she put Cathy in the "who referred you?" box.)
If you did not do this when you registered, scroll to the bottom of the screen and click the Contact Us link. Submit a feedback message giving your referer's name and city/state.