YOU CAN STILL REGISTER YOUR AMEX CARD!
I just called the Amex Customer Service phone number (1-800-528-4800)
and told them that I had forgotten if I signed up for Small Business Saturday, but they told me that they could sign me up for it by phone and did.
Update: I forgot to mention that this only works for accounts you have directly with Amex, they didn't allow me to do it with an account I had with Citi .