There's some kind of problem on Digital River that, if you start from an empty cart and place a Bundle product, the "Redeem Coupon" field doesn't show. They are looking into it but, in the meantime, the solution is:
- Empty the cart
- Put another product in the cart that is not a Bundle, like a scenery. The Redeem Coupon should now appear
- Add the GSX Bundle
- Remove the other product
- Insert the coupon code.