Method 1 (Arc Client)

Download and log into the Arc Client
Click on the gear icon located on the top right of the client
Select “Activate a Product” and enter your code
Log into the game and complete the tutorial if you have not done so
Reward can be claimed at the Rewards Claim Agent in Protector’s Enclave

Method 2 (website)

Go to https://www.arcgames.com/en/redeem/ (make sure you are logged into the website!)
Enter code in the Code field and Submit
Log into the game and complete the tutorial if you have not done so
Reward can be claimed at the Rewards Claim Agent in Protector’s Enclave
 

Redeeming codes on Xbox One and Playstation®4

Go to https://www.arcgames.com/en/code-redemption, and follow the instructions on that page
Link your PSN/Xbox account to an Arc account, if you haven't already.
Log in to your PSN/Xbox account on a Playstation®4/Xbox One Download and install Neverwinter
Begin a new character and complete the tutorial if you have not done so
Reward can be claimed at the Rewards Claim Agent in Protector’s Enclave
 

Redeeming a Playstation®Store Voucher:

Open a Sony Network Entertainment account on PSN (or use your existing Sony Entertainment Network account).
Select the PlayStation®Store icon on the PS4™ system home screen.
On PlayStation®Store, select 'Redeem Codes' at the bottom of the menu.
Enter the code.
Once the code has been entered correctly, select 'Continue' on the dialog box.
Select 'Continue' to complete code redemption.
Launch Neverwinter
Reward can be claimed at the Rewards Claim Agent in Protector’s Enclave