Attendee fields are enabled by default in Tickera settings. However, it happened more than once that customers disabled this by accident and immediately they reported that something's not working right. So, if this happened to you as well, please navigate to Tickera Settings -> General tab and make sure that an option named Show Ticket Attendee Fields is set to Yes. This also affects the use of Custom Forms add-on and if you have created a custom form but it is not displaying on the cart page, make sure that you haven't disabled attendee fields.
Now, if you would like to collect email addresses of each attendee, navigate again to Tickera Settings -> General tab and set Show E-mail for Option For Ticket Owners to Yes. This is also mandatory if you want to allow ticket delivery to each of the attendees instead of all the tickets being delivered to the buyer email address.