I just got my NAS, first one, and this is my first time for me setting up my own FTP.  I followed all the instructions for setting up my NAS, and I still couldn't get it to work.  
I created my own DDNS using D-Link's free service, and I forwarded port 21 on my router and still no dice.  I did a little searching and I found that my ISP, Charter Communications, blocks port 21 in order to prevent customers from setting up their own servers.
My setup right now is:  Ubee cable modem -> Linksys WRT45G -> Netgear gigabit switch -> NAS    
WRT45G ConfigurationLogin Type:    Automatic Configuration - DHCP          
IP Address:    71.81.77.7          
Subnet Mask:    255.255.240.0          
Default Gateway:    71.81.64.1          
DNS 1:    24.217.0.5          
DNS 2:    24.217.201.67          
DNS 3:    68.113.206.10          
MTU:    1492
DNS-321 ConfigurationIP Address:     192.168.1.107
Subnet Mask:     255.255.255.0
Gateway IP Address:     192.168.1.1
Mac Address:     F0:7D:68:69:22:F7
DNS1:     24.217.0.5
DNS2:     24.217.201.67
<a href="http://s1106.photobucket.com/albums/h362/davwils/?action=view&current=stuff.png" target="_blank"><img src="http://i1106.photobucket.com/albums/h362/davwils/stuff.png" border="0" alt="WRT45G Port Forwarding"></a>This is what the port forwarding page looks like on my router.  
From what I looked at on other threads people discussed opening some public port > 1023 and have it forward to the private port 21, or using PASV vs Active.  I didn't know what this meant, or if I could even accomplish this with my current hardware.  The D-Link FAQ page was pretty vague about any of this, and all it said was I could use nonstandard ports and that I would access my NAS externally by typing my DDNS page into the URL. 
From what I understood mine would be:  spacecommand.dlinkddns.com:XXXX  with XXXX being the new port that my router is listening to for the FTP server.
And yes I have my router configured for DDNS with my webpage.
Any help on this would be greatly appreciated as I am tired of reading mass quantities of forum posts looking for an answer.