squidman,
Below are the steps you need to take to make it work. It would take a hugh post to cover all the details so I'll give you the summary so you can decide if you want to try it. If you are willing to give it all a try then take it one step at a time and ask for more assistance as needed:
1) Your router is likely assigned an external (Internet reachable) IP address by DHCP from your ISP. However, this IP can and will change. Therefore, to be able to even find your router on the Internet you need a reliable way refer to your router no matter what it's IP is. To do this you need to set up what is called Dynamic DNS (DDNS). To do so I recommend going to
http://www.dyndns.com/, reading up on DDNS, then signing up for a free DDNS account and domain name, then downloading and installing the client, then setting it up on your Mac.
2) On your Mac, turn off your firewall completely while testing (to remove yet another layer of complexity). You can turn this back on after you get it all working.
3) On your Mac, turn on "Personal File Sharing" in the Sharing preference pane.
4) On your router, you will need to either put the Mac you want to reach on the "DMZ" or enable port forwarding to its private (internal) IP address for ports 548 and 427.
5) Lastly, I would disable the firewall on your router as well until you get it all working, then reenable it later.
Unfortionatly, making a remote connection to your home Mac's files through the Internet is not a trivial task considering all the steps needed. However, if you take it one step at a time I'm sure that you can get the help needed on this Forum.
Tim
17 PB G4, Xserve G5x2, XRAID 2.5TB, 20 iMac G5, 17 iMac G4 Mac OS X (10.4.3)