It works like this:
You choose the add-ins you like and add them to your cart.
Click ‘Checkout’ and provide your credit card information and email address.
You receive an email a few minutes later describing how to download the add-ins along with your order number, which is used to activate the add-ins (that is, go from trial mode to fully functional mode).
Therefore, be sure to provide a valid email address when you order. Note that there will be two, one for billing (receipt) information, and one for the order (download/installation instructions).
If you’ve downloaded the add-ins already, it is not necessary to uninstall and re-install. Simply restart Outlook, wait a few seconds for the trial mode window to pop up, and then click the “I bought it” button. Provide your order number and you’re all set!