Discount (Coupon) codes can be applied to an order in the POS system by selecting the percentage icon in the left sidebar of the system. Once clicked you will be shown a pop-up that displays all enabled coupon codes in your Magento system:

Can I create a new discount code from the POS?

At this time discount codes can only be managed in the Magento admin panel.

How do I manage the discount codes shown in the POS?

The POS will show all coupon codes that are enabled in your Magento account. To manage the discount codes for your store please go to Promotions -> Shopping Cart Price Rules