Adding a new course to your account with unused credits is very easy. First make sure you are logged in. Next click Add Course. If you only practice in a single state you will go directly to your state's course list. If you practice in multiple states, select a state from the drop down.
Under the state name you will see how many unused credits are in your account. You can add any course without charge as long as that course isn't more credits than you have in your account. Find the course you want to add and click the blue Add Course button.