To purchase courses, click the "Courses" link on the left side. You'll see a list of all your unused courses and a button to purchase additional courses. Click that button and adjust the number of courses you'd like to purchase on the right. As you change the number of courses, you'll see the discount and total cost update instantly. The last step is the checkout, which is where you'll enter your credit card information or choose a card that you've already used.
Once the courses are purchased, you can then enroll employees in the courses.