you have to set it up in your router,usually called something like virtual servers. You point a particular IP port (for FTP it's 21) to an internal IP address.
<i>It's always the one thing you never suspected.</i>
Your ftp address is going to be your IP address. or what ever your dns is if you pay for that.
to log on to it just type this into internet explorer ftp://usernameassword@4.43.254.206
(replace username and pass with whatever urs is)
If you have dynamic ip assignment then when ever you reconnect to the internet the ip at the end will change. If you are paying for a dns address then use that instead of the ip address.
-Folken