If you do not have any available store credit, information about store credit does not show up on your account page or as a check out option. However, if you do have available store credit it will display predominately on the HOME page (and others) in the top, right-hand corner.
You can also see your available store credit from your account page. Log in to your account and click on the ORDERS tab. You will see your available store credit displayed in a box just above your list of orders.
When you check out, the option to use store credit is assumed and already checked. If you do not want to use your store credit, unclick the checkbox and choose a method of payment.