|I have disabled FTP transfers because of the lack of security with this NAS.|
On a LAN this is kind of moot...unless you frequently allow strangers into your home (or place of work) and allow them to use your computer(s) without monitoring them. You could look into sftp (secure ftp) if you're really worried.
If you access your LAN, and the NAS remotely and that's got you concerned about the security of ftp then you're barking up the wrong tree anyhow. You should be concerned with creating an encrypted VPN tunnel to your LAN from outside. If you do that, you could do plain text ftp type transfers without worry as they would be inside the encrypted VPN tunnel.
Having no experience with that NAS unit I can't say for sure what your issue is. However, I'd wager I could buy one, plug it into my LAN and have it up and running perfectly in no time flat. I say this because as a computing professional, I always read the instruction manual before attempting to use a new (to me) piece of equipment.
Something tells me you haven't quite read the entire manual or you'd know how to create users/groups on the NAS and administer them correctly.
Stop what you're doing, reread the entire manual (not the 'quick setup guide') carefully and then take another crack at setting the NAS up. I wager if you do that, you won't have need to come back here and ask for help as the manual likely contains all your answers if you would but read it....