The site that you will use to edit your registration site is experience.eventmobi.com. If you do not have access to your registration site editor please contact support so they can add you as an organizers.
Once you login with your EventMobi account, all available app registration sites you can edit will appear. Select the app you would like to edit, select your registration site and open "Registration" from the left hand menu under "Products. The "Registration" tab will now expand to show all available options for editing that you have on your registration site.
If you would like to view your registration page you can access it by going to register.eventmobi.com/YourShortcode. Additionally you can click the "Dashboard" tab and then "View Your Registration Site" in the top right corner.