ADD A COUPON
Go to Inspections --> Scheduled Inspections (or Unscheduled Inspections):
From here you can click on Actions -- Apply Coupon (or click the Coupon icon if you're using the icon view)
Alternatively you can click the 'i' Icon to go to the order info screen:
And click Actions --> Apply Coupon:
The coupons that you have set up in your ISN will appear under the Apply Coupon drop-down, which will also allow you to select what fee the coupon is to be applied to:
Hit Apply Coupon and you're done.
REMOVE A COUPON
Clicking the 'Apply Coupon' option again will allow you to Remove Coupon or add another one.