Your router will hand out the IP addresses, assuming that your computers are set to obtain IP addresses automatically. The router itself, as it faces outwards to the web, has only a single address.
I'm assuming that you, on some computer sitting behind the router, are going "out" to get something. In that case you shouldn't need to do anything at all about ports.
But if you're expecting somebody on another computer to contact your modem/router and then pass through into one of your computers and leave something there, then that's a different matter. The most common, and easiest, situation is when it's you who are going out to get something -- meaning that you initiated the contact. In that situation, Fetch should do everything you need.
As for the iMovie thing I can't help except to guess that an iMovie project has different sorts of files in it -- maybe some text, some binary. If that's the case, have person with the iMovie project zip it up (stuffit or equivalent). The zipped up file will be all binary. Then at your end YOU (using Fetch) go and, well, fetch it. Use the "get" command. Once it's on your Mac, unzip/unstuff and -- most likely -- you'll have what you want.
Bottom line is that it's way, way easier to go out and get something and pull it back than it is to have somebody else push it to you.
Hope this helps.