Yes. Currently the button at the bottom of the page says "Stop FTP server"
Actually, I have not yet tried to access the FTP server from the LAN. Will try when I go home.
No, I am trying to access the NAS from my work computer.
The easiest way to check if TCP port is opened is to go to command prompt and issue following command:
telnet X.X.X.X YY
Where X.X.X.X is the IP addess of destination and YY is the port number (21 in your case)
If the port is really opened you will see at least black screen (in general case) or some text reply in case of HTTP, SSH, FTP, SMTP and many other protocols. If port is closed you may either get error saying that port is closed, connection is reset or just timeout error if there is no device at this address or firewall/device just silently dropped your packet without sending notification back (i.e. ICMP port unreachable).
BTW Are you sure that port 21 is not blocked from inside your network at work or by local firewall on your work PC?
It is always a good idea to test connectivity to NAS locally (from the same network where NAS is installed). IF it works, but access from the internet fails, check your router config and log messages once again.
Can your provider block FTP traffic?