To the right of Registration Status set the drop down box to either Open, Closed, or Scheduled to open on a specific date. This will determine if your form will be available at all for online registration.
Note: If you select scheduled to open on a specific date, you will be able to enter a message to display to registrants as well as a date/time to open registration. Please be sure to Save the form and click the 'Publish Current Version to Link' button to save your changes.