Generally, in order to attach a coupon to a Mass notification campaign, the campaign needs to be Dynamically generated and the "number of guests" should be set to 0.
If the number of guests (while configuring the coupon campaign) is set greater than or equal to 1, the coupon will not populate in the mass notification campaign.
This is because the number of guests who will receive the coupon has not been specified in the mass notification campaign.
NOTE: Expired coupon campaigns do not show in the drop downs either. Make sure you are looking for a valid campaign with a current coupon