The best way to add an applicant is to first set up a new enquiry entering first name, last name and email. Then, from the enquiry list action button select "invite applicant". See the enquiry section for full details.
Once the enquiry has completed the application form online, the person will have become an applicant and will be displayed in the applicants list.
You will also receive a notification when an application is completed online.
Alternatively, if you really need to manually add a new applicant (but there should never be any need to), you can do so by going to the list of letting units and from the actions button click "add applicant".