You need to create a sub-forum to which only "Members" have access and post your files there.
Do you mean "Members"? This is a different group to "Registered". Any user can register, just by entering their email address. They will be assigned to the "Registered" group automatically, but you need to assign your forum users as "Members" manually.
As you are using the "Mixed" application, it is also possible to create part of your forum so that only "Members" will see that sub-forum, so you can make it completely private.
For which of these things do you need instructions?
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.