The appconfig in your version is called DisallowCoupons. When set to true, coupons cannot be used. One of the other things you should check is the customer levels you have set up in your store. Each customer level has an option to allow or disallow coupons (in the database this field is called LevelAllowsCoupons in the dbo.CustomerLevel table) and when set to 0 (disallow) any customer belonging to that customer level will be unable to use coupons.
<a href="http://www.aspdotnetstorefront.com">Shopping Cart Software</a>