You must have installed AdsManager and Invoicing installed, to check if they are installed, click on "Component" link in the top menu, you must see both "AdsManager" and "Invoicing". Click on the "Coupons" tab, click on "New", you will see list of options of AdsManager. You only need to click on the option you want to, for example, the "Renew" option and validate your coupon. This coupon will only applies on the ads renew from now. You can also chose many types by maintain the CTRL key pushed and click on the types you want.