First you will need to Login to your account in the clientarea and select My Products & Services
Click where the red arrow is (on the green right arrow at the right of your product) then you need to click
» Account Addons - Click to view available addons as shown below.
The next screen displayed will show you a list of Add-On's available for your product, Select the Add-On you need and click Order Now to complete the purchase.
The package above is an example only, when you press the according buttons you will see your packages (active and previous if you have any).