Taken from the offer page:
Dead Island Pre-Purchase Information
Customers who pre-purchase Dead Island: Riptide will receive, via your registered account's email address, a single-use voucher to allow a 100% off purchase of Risen (Steam activated version)
In addition to this, customers will also receive the Dead Island: Riptide Survivor Pack DLC, providing a BBQ Blade, an XP / Stats booster, and a special shop in game!
When will I receive these keys and vouchers?
You'll receive an email containing your single-use voucher for Risen by the end of next week. Importantly, when you use this voucher, ensure you are logged in to your GMG account.
Please note that Dead Island: Riptide keys and Survivor Pack DLC keys will be distributed by the day of release.
If you are a new customer, you (and me) could get another $2 store credit using my referal http://www.greenmangaming.com/?gmgr=katukeci
Where does it mention the $10 cashback?