I think you need to read the section of the manual headed - Section 4 --> Configuration --> Account Management --> users/groups in your language.
If the 'administrator' are talking about is the administrator that is allowed to change the settings of the DNS-320L then there is no way that you can see individual files no matter where they are.
If, on the other hand you want to be the administrator o***roup of users then you need to set up a grout and include the users in it with the administrator, as a user, having full access (see the manual section above).