If you already have an existing membership (NB. since October 2008 using this new system) then you can add additional products to your membership.
Log in to your Control panel using your username (lower case) and password (case sensitive). You should now see on the left your existing subscriptions and on the right an area where you can add or renew your subscription. Select the appropriate product from the drop-down menu, enter a coupon code if you have one and click on 'Order'. You'll then be taken to a Paypal page where you can make your payment.
Having processed your payment you should now have access to the new product through the Select Course button.