After installing the Coupons add-on, be sure to check the add-on settings.
To access the add-on settings page, find the add-on in the My Apps section and click on the Settings link.
The add-on settings page opens.
At present, there is just one option available on this page:
Always show coupon code box: Defines whether the coupon code entry box must be displayed by default at the checkout and in the cart. If the setting is enabled, the box is displayed. Otherwise, the box is hidden, and the customer needs to click the "Have a coupon code?" button manually to make it visible.