I don't use the feature to see to retailers, but to use the coupon code, just login and goto the Dashboard or My Account page, and click on the title of your approved and live book.
Scroll down and there will be a section for the discount codes. Click to add one.
The first time (and any time you need to add a new code), you'll need to click the link that's in the instruction paragraph that will create the code for you.
Then, copy and paste the code it created, and go back to the codes page, and enter it into the appropriate box. Next to that, you can select whether you want the code to be a percentage or dollar code. So, if you want 30% off, you'd put in percent in the drop down and 30 in the other box.
For $5 off, you'd put in dollars in the drop down and 5.00 in the other box.
That's it. Then, give the code to whomever you want to use it. If you have a blog or website page, especially one created just for the retailers to order on, you can include the code there with the createspace link.
The great thing about the orders on your Createspace store is that they show up on your reports right away, so you'll know if the store ordered or not.
I'm not sure if the coupon codes work for multiple copies, but in case they do, I'd use a percentage off coupon. That way, if they buy 5 books, they receive say 30% off the entire order...whereas if you use a $5 off or something coupon, they may only receive $5 off total, not on each book.
Hope this is what you were asking and that it helps.