You can add coupons to a promotion in two ways:
On the General tab of the Coupon, you can select an existing promotion or create new promotions with which to associate the coupon.
On the Availability & Coupons tab of the Promotion, you can select existing coupons or create new coupons with which to associate the promotion.
Click a display name to view or edit details of the promotion or coupon, depending on your starting point.