To redeem your reward, you'll first need to create a free account using the Sign Up button at the top right. Then, head over to the Modules and Subscriptions section of your profile. There you'll see a Redeem Rewards link. Click that, and under the section titled InfoStack Key, enter the key you received from InfoStack for this reward. Finally, click Redeem Reward to unlock your free access!
If you run into any trouble at all, don't hesitate to contact us.