http://www.americanexpress.com/openpayovertime13 or call 1-800-492-3344
From the mailer:
As a Business Gold Rewards Card Member with an excellent payment history, you're invited to enroll in the
Extended Payment Option. With this feature, you can pay for eligible purchases of $100 or more in full or over time.
And after you enroll your Card in the Extended Payment Option
you can receive 10,000 Membership Rewards points.
No fee to enroll