Merchants who offer coupons are treated as a separate merchant.
For example, if Best Buy offers coupons, there will be 2 listings:
Best Buy (products)
Best Buy (coupons)
Merchants who have coupons are listed under their respective affiliate network in the factory, but the affiliate network name with have "Coupons/Deals" next to their name. Here's an example of what I mean:
So you should go into those networks and select the merchants whom you want to show coupons from.