stolen from http://www.game-warden.com/forum/showthread.php?t=267
and modified a bit.
How to get Freespace 2 Multiplayer working NOTE: These instructions assume that you have a fresh freespace 2 install. If you have any previous modifications or campaigns installed there is a good chance that this tutorial may not work properly for you.
By: MatthewPapa (edited by Turey)
1. Firstly go register a multiplayer account here.
Be sure to write down/remember your multiplayer account information!!
2. Download FreeSpace Open here
3. Click on Start>All Programs>Games>FreeSpace 2>FreeSpace Open Launcher 5.4. Go to the MOD tab and hit No MOD. You may adjust all the other settings in the launcher to your preference, but be sure and leave networking ON. Click apply when done.
4. If you are behind a router or firewall you will need to forward port 7808 both UDP and TCP to your computer. It is critical that you do this or other people WILL NOT be able to join or even see your games
! They may show up on the site, but people WILL NOT be able to see or join them in the game. Consult your router/firewall documentation for more info on forwarding ports. It will probably look something like this: IF THIS IS NOT DONE PROPERLY, IT MAY SERIOUSLY DIMINISH YOUR FS2NETD EXPERIENCE!!
5. Open up Freespace 2 via the launcher by clicking run. Tab over to multiplayer on the choose pilot screen and create a new pilot. It is preferrable that the pilot that you create be the same name as your account name that you created earlier (avoids some confusion for new people.)
6. Go to Freespace 2 preferences now and then go to the sub-tab "multi preferences." Select PXO and then type in your multiplayer username and password (the one you registered earlier.)
You can safely leave the "squadron" field blank.
7. Pilot, you are now qualified to fly FS2NetD! Good luck. Click on the main door to host or join your first multiplayer game!
Extra files that you might need later when you get better:
Working table files. If you are getting the message "you have hacked tables" then unzip this in the \data\tables\ directory. File
Valid multiplayer missions (ones that cound on your score) File1
Thanks to MatthewPapa for 99% of this post.
EDIT: Stupid differences in URL tags. Fixed all of them.