There are two steps you need to follow when setting up a coupon:
1. Add the coupon [Admin: Add Coupon]
2. Assign the coupon [Admin: Assign Coupon]
The coupon cannot be used until you have assigned it to an individual or group for which it is valid. The 2nd step is normally missed when setting up coupons and then they won't be available for use on the site.
Once the coupon is assigned it will then work on the site.
Tip: Remember you can quickly find a program by typing in words into the Admin search box, e.g. coupon