Ok, what this most likely means is that you have a dynamic IP address as opposed to a static one. An IP address is exactly what it sounds like - a unique location that receives and sends data. Every computer has its own IP address that is it's internet ID, basically. The problem with routers is they use dynamic IP addresses, meaning that they give you a temporary IP that you use for a while, then they change it about every month or so. They do this because it allows you to hook up a router to any computer and get connected without much setting up required.
You need to assign a static IP that doesn't change; otherwise, you would have to go through this procedure everytime it was switched. Unfortunately this is not a quick thing to do and I'm not the most qualified person to be explaining it all to you. Here's the guide I got all my answers from, hopefully it helps:
When (and if) you get around to doing all the steps it goes through, you will have a shiny static IP address and then you can go here to do the actual port forwarding:
This whole process is pretty time intensive and took me several hours to get all worked out (though I'm kinda slow:)), so don't get discouraged. You may even learn some useful techno babble while you're at it.
Disclaimer: If this does not solve your problem...well, sorry for wasting a crapload of your time:)
