When trying to add individual users to a map point policy, Active Directory user accounts which do not have a User Principal Name configured are not returned when you search for the username.
In Active Directory, there are three accounts - 'blah', 'blah1' and 'blah2'. Only 'blah' and blah1 have a UPN (shown in the 'User logon name' section).
When searching for 'blah' in the Select User box you only see users returned which have a User Principal Name:
Ensure that any accounts which you want to use with File Director have a UPN assigned.