To use your store credit you will need to activate your account associated with your email address if you have not created an account when checking out. You will receive an email to activate your account if customer service has provided you with a online store credit. Simply click the link in the email and create your account then login.
Once you've logged into your account you can view your online store credit balance by clicking the person icon on the top right menu button. This will bring up your My Account page which will indicate your online store credit balance which is active on your account.
To use your credit simply add items to your cart then display your shopping cart and click the "Apply store credit" button found at the bottom right of the cart above the "Checkout" button.
When store credit successfully applied to your cart the following message will be displayed. Please continue to checkout.
Your store credit will be applied as a discount code at the checkout and will display like the below. Please complete the checkout and make payment. Your order will be complete.