If you wish to keep the program, you register by sending a nominal fee as specified in the program documentation.
The process of registration is simple, you either mail a check/money-order to the address specified in Contact page or by registering online via secure transaction (service provided by RegNow).
At this time, not all programs are registerable online, however the most popular ones are. Click on the program you're interested in (on your right pane) and see its documentation.
Once a program is registered, you will promptly receive all information reserved for registered users. This will include a registration key that revives the expired program(s). Make sure that you provide your email address with every registration so that you can receive this information fastest.