Navigate: Main Menu > Configuration > (middle menu) Checkout > Payments > (Scroll down to "Debit and Credit Cards") Click "Connect Single" > Scroll down and select the checkbox for "Network Merchants Gateway"
You'll need to obtain your account credentials from Network Merchants:
A: Her's the Network Merchants Gateway (N.M.I) Response Code Lookup Table:
Refer to this list when reviewing the transaction history of an order.
A: Typically merchant accounts want you to only send funds back to a card that was originally charged. If they allow you to use credit then you can send funds to an arbitrary card. That is a HUGE potential for fraudulent abuse. That being said, when you migrate from one merchant account to another, there can be a 30-60 day window at the start where you might need to have that enabled so you can refund arbitrary cards.