(You know, what you want to do would be a lot easier with a common drop-box server - e.g. an
iDisk or
Yahoo! BriefCase.)
It looks like that ADSL Router indeed uses Network Address Translation (NAT) services in the ADSL Router (which is generally a good thing for sharing a home network.) With the default configuration, you are apparently using DHCP to assign an IP address to each Mac or PC in your home as it connects to the ADSL Router. This means each time your Mac wakes up to boots it can get a different IP address (seen only on your local network - the Router presents an entirely different IP Address to the outside WAN/internet, which is the one WhatIsMyIP.com shows you.)
Your ADSL Router is helping you share your internet service among all the Macs and PCs it's connected to by translating the local IP addresses of each Mac/PC to the external IP address assigned by your ISP. (To see this distinction, go to the Network System Preference pane, and look at your Mac's current IP address on the TCP/IP tab and compare that to the address WhatIsMyIP.com sees. All the Macs and PCs on your local network are known by that same external (WAN) IP address on the internet, but each has its own unique IP address on the local network.) The ADSL Router performs this Network Address Translation by keeping track of every connection your Mac (or PC) opens. It swaps the IP address for each connection so traffic is always routed to the correct Mac or PC.
The complexity arises when some external internet site tries to connect to your Mac (such as happens when running an FTP or web server on the Mac, gaming, or file sharing). When the ADSL Router receives the connection request from the internet, it doesn’t know which Mac or PC on the local network to pass the connection on to; so it gets dropped.
To rectify that, you need to tell the ADSL Router which Mac gets each incoming connection, based on the connection's port number. You'll need to assign the Mac that will support FTP server a static IP address (so you have the same IP address all the time) and then tell the ADSL Router to pass arriving connections for the FTP ports on to the static IP address you assigned the Mac with the FTP server.
(Caution: Keep in mind in all the following I just scanned your manual and have never used this Router. I filled in my own assumptions. Obviously I haven't walked through this for your modem or ISP. Your results may vary.)
Open the Network System Preference on your OS X Mac.
On the "Location" pull-down, select "Edit Locations...". Then select your current Location name and click "Duplicate". Edit only the duplicate entry and it will be really easy to switch back to the old Location configuration if things don't work out.
On the TCP/IP tab, change the "Configure:" from "Using DHCP" to "Manually". Set your IP address to something unique and outside the range the ADSL Router is dynamical assigning, e.g. 10.0.0.100 (I picked this address because per the manual the default range of DHCP-assigned IP addresses allocated by the ADSL Router is in the range 10.0.0.3 to 10.0.0.14, in the DHCP section of LAN Configuration web page, manual section 5.3. This address should be outside the range of those addresses assigned by the ADSL Router's DHCP server) set the subnet mask to 255.255.255.0, and the Router to 10.0.0.2 (That's the default address the ADSL Router offers on the local network, according to the manual.) Enter at least one DNS (Name server) address your ISP provides. Click Apply, Disconnect and re-Connect your Internet Connection, wait a minute, and make sure your connection to the internet still works. (If not, switch back to the old Location to restore your previous settings.)
Then configure the ADSL Router: (Remember I'm suggesting this from a scan of the manual only, not actual experience). Launch your browser and visit the ADSL router Admin with the URL
http://10.0.0.2 (per 5.1 on the manual); enter the userID and password.
Click LAN under Configuration (left side), (per 5.3 of manual). Under DHCP server examine the values for "User-defined Start Address" and "User define End Address". The manual says the defaults are 10.0.0.3 - 10.0.0.14. (The address you assigned to the Mac, 10.0.0.100, should be outside this range.)
Set the port mappings. Click "Virtual Server" under Configuration. (per 5.5 of the manual): Fill in the line with the next available "ID" number (e.g., 2) and set both public and private port to 20, pick the TCP box, enter 10.0.0.100 (the Mac's IP address) as the Host IP Address, the click "Add this setting." Repeat for port 21: Fill in the line with the next available "ID" number (e.g., 3) and set both public and private port to 21, pick the TCP box, enter the same the Host IP Address, the click "Add this setting" again.
If I read the manual right and made the right assumptions, your ADSL Router should now relay incoming connections for an FTP Server to the Mac. (I didn't see anything about needing to reset the modem for the changes to take effect.)
Now you need to turn on FTP Services on the Mac.
ALL YOUR Mac's ACCOUNTS SHOULD HAVE GOOD, HARD-TO-GUESS, NON-DICTIONARY PASSWORDS. Your FTP server is going to be visible to everyone on the internet. There are unscrupulous and malicious people out there who randomly probe around looking for FTP servers and try to guess account passwords to get into your system. Don't leave your system wide open to them, or you'll become a server for any number of unethical and illegal activities.
Now on the Mac you can turn on FTP Access (Sharing pane of System Preferences), Services tab. When you click Start for that, it should automatically open ports 20-21 in your Firewall, but it doesn't hurt to check the Firewall tab to make sure FTP Access is checked as an exception there.
An external system should be able to connect via FTP to your system using the IP address given by WhatIsMyIP.com. (A system in your home would use the static IP address 10.0.0.100, which wouldn't test the router config.) The FTP userID and password will be the ID and password of any user account account on your system. If the person connecting isn't you, I suggest loggin on as Admin and using the Accounts System Preferences to make a new, non-admin user account for that person, giving it a good password, and telling them what it is.
Hope that helps.
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