I got the following email from audible last night
"Hello, Thank you for contacting Audible!
We're happy to hear you've enrolled into a Free Trial of our service. As a thank you, we've provided you with a promotional coupon to use towards the purchase of select Hardcover and Paperback books sold and shipped by Amazon.com.
This promotional coupon will automatically be applied to your account and will appear in the "Order Summary" section of the checkout page. You should see a discount labeled "Audible Promotion" showing the total discount applied.
Please note that it may take up to 24 hours for the promotional coupon to appear in your cart after redeeming the Audible Free Trial offer.
Thank you again for contacting Audible. If you would like additional assistance, please do not hesitate to contact us. We are here to help 24 hours a day, 7 days a week."
So I'm guessing it should be coming shortly