You can't add coupons through WSI the way you do with products or customers, we just haven't implemented that yet. You'd need to use the ExecuteSQL node to do a direct DB insert - something along the lines of:
Code:
<ExecuteSQL Name="string">
<SQL>
<![CDATA[
INSERT INTO Coupon (CouponCode,Description,StartDate,ExpirationDate,DiscountPercent,DiscountAmount,ExpiresOnFirstUseByAnyCustomer,ExpiresAfterOneUsageByEachCustomer,ExpiresAfterNUses,RequiresMinimumOrderAmount,Deleted,CreatedOn)
VALUES(somecode,somename,getdate(),getdate()+30,10,0,0,1,5,0,0,getdate())
]]>
</SQL>
</ExecuteSQL>
*Please ensure that you have a full, functioning backup before making any code modifications or file changes to your AspDotNetStorefront website, or running any ad-hoc queries against your database. Improperly making code modifications or running queries against your database can cause your website to be non-functional and/or your data to be permanently lost. As part of your disaster recovery plan, you should make regular backups of all files and data, and perform periodic checks to ensure your backups function properly. If you are not sure if your data is adequately protected, contact an IT professional or your hosting provider for assistance.