The link on the groupon didn't work. So here is the answer, straight from tech support at ARe:
"I’m happy to help! To redeem the groupon code you received, you need to log into your account and go to
http://www.allromanceebooks.com/groupon. Enter the code into the text box and select “Redeem”. The amount of the groupon purchase will be automatically credited to your account and show as “Available Ebook Bucks” at the top of your Library. To use the bucks to make a purchase, select the “Pay with Ebook bucks” button on the payment page."