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 for it.
The add-on settings page opens.
At present, there is just one setting available on this page:
Always show coupon code box: Defines whether the coupon code entry box needs to be displayed by default at the checkout and in the cart. If the setting is enabled, the box is displayed; otherwise, the box is not displayed, and the customer needs to click the ‘Have a coupon code?’ button manually to make it appear.