This issue usually occurs when there are no attributes with a record type of Event in your database. To add an attribute with a record type of Event follow these steps:
  1. Click Administration > Attribute categories 
  2. Click the add button
  3. Change the record type to Event
  4. Finish creating your attribute and click save
Similarly if you don't have an attribute with a record type of registrant in your database the attribute tab will not show on the registration record of a registrant that is signed up for the event. You would need to follow the steps above but in step 3 change the record type to registrant to resolve this issue.