Add a new field to registration
To add a new field that you created on the Contact object to be displayed on the event website you need to expose that field to the site, by default Salesforce restricts visibility of objects and their fields and we have to manually 'opt-in' to being displayed on the website. Luckily it's easy to do.
Scroll down to field level security
Locate your field that you want to expose and check the checkbox.
Then hit save. And you're done.