Two things, the user you have selected is fully defined in users/groups section, and you have an FTP client installed on one of your computers.
The user will have a user name and password which may, or may not, the same as you used to log on to the unit.
Using an FTP client (e.g. Filezilla - free from web) will allow you to see the full connection handshake process and should show any problems.
If you do use FileZilla the host is the fixed IP of your NAS, username is the name you defined in user/group (it is case sensitive - Name is not the same as name), password is the one you defined for the user and port can be left blank (defaults to 21) unless you changed the default. Clicking on Quick connect will then start the connection process which is shown in the upper information section.
There will be a pause after the 'waiting for welcome message...' is displayed because the NAS has to reply. You should then see
Response: 220------ Welcome to Pure-FTPd [TLS] ---------- followed by more messages until you get to,
Command: USER xxxxx (where xxxxx is the user you defined)
Response: 331 User xxxxx OK. Password required
Command: PASS ********
Then if all is well you get.
Response: 230 OK. Current restricted directory is

(with

being the directory defined for the user)
After several more messages you should get.
Response: 211 End.
Status: Connected
At which stage you should see the directory listing in the Remote site (on the right) list pane.
Until you can connect on your local network it is no good trying to connect from outside.