If its working internally (on LAN) it should also be working on the internet provided the server running HFS has the required ports forwarded thru your router/firewall and you know how to gain access to the server with the correct IP address format, which is a confusing method for a newbie, because internal and external IP addresses for the same machine are different, So i would suggest setting up a dynamic name to your server which will work internally and externally.
Try set up and install dyndns on your server pc to direct requests to HFS, it is a simple application that will strap your server machines IP address to an internet name such as somename.dyndns.ws once this is set up and working u should be able to access your server externally and internally using the dynamic name set up by dyndns instead of using the ip address to point the request to your server.
I notice HFS has got set up options for dyndns however i have not configured them as dyndns was already in place and working on my server prior to HFS install. (i have tried a few servers and HFS is a top class package)
If your internet account has a dynamic IP address you will want dyndns or similar anyway so your server can be found by external users after its IP address changes, as a dynamic IP address can change at any time hence the use of the dyndns client keeps track of your constantly changing IP address and keeps it bound to the dynamic name you have given it.
In short HFS is prolly working fine both internally and externally already, its just not answering incorrectly formatted IP addressing. Give your server a dyndns name, access it via the name not the IP address....
Go here
http://www.dyndns.com/ start a free account and set it up, download and install the dyndns client application to your server, it will ask you to sign in to your already created dyndns account then presto that machine now answers requests directed at the name you specified in dyndns account set up, obviously HFS will also need to be running as it will answer the request not dyndns, dyndns will only make sure the request lands at the target machine.
The dyndns update client (which runs on your server) was a touch hard to find on their site some time ago when i signed up but is required for it to work....
Hopefully this helps...Cheers