If it is inside the linuxmce Net you should open port on the core
go to webadmin Advanced > Firewall rules And create a rule there for it
on your router you should open the port for the linuxmce Core not for the actual NAS
Hope it helps
Actually it depends on a number of things - FTP for a start off can be a pain across NATs and firewalls depending on whether it is passive or active. And the advice above will only work in certain circumstances - there are other ways of doing this. If this is still an issue, let me know and I will try to help.