To apply store credit to your order, select "Pay using my store credit" during Step 4 of checkout. If you have store credit available, this option will be automatically selected for you. Even if the amount of store credit on your account is not enough to pay for the entire total, keep this option selected. Then, go to the bottom of your screen and click "Proceed to Payment" to get to the next step, where you'll enter your payment information for the remaining balance!
If you select "Pay using Credit Card" during checkout, your store credit will not be applied to that order. However, your store credit doesn't expire, so if you don't use it on an order when it's available, it will still be in your account to use on your next order!