You can sign up for Megavolotea during the booking process. You will find subscription details in the second page of the booking process, in the contact section.
You can also activate your Megavolotea account from your user profile. There you will find the Megavolotea section where you can subscribe for the service.
In addition, you will also be able to subscribe to Megavolotea from the page that appears in http://www.volotea.com/en/megavolotea.
Remember that you can enjoy all the benefits of Megavolotea for 12 months from the date you sign up for the program.