By clicking on Add / Drop Classes in the menu bar, the registrar i'll have the list of applications for adding or dropping classes as shown in the below figure.
By clicking on the open button he will have the application details as shown in below.
Here he will have two buttons as approve and Reject. If he wants to approve he can click on the approve button or if he wants to reject the application he can click on the reject button.
When he clicks on the approve button he will have the success message as shown above.