Assumed that you're running windows vista/XP(>SP2) - Do this:
CONFIGURE ROUTER
1...START -> Run -> type "cmd /k ipconfig" -> OK
2...Look at your internal IP address (In 95% it starts with 192.168.XXX.XXX)
3...Route port 8303 to your internal IP address (refer to your router manual)
CONFIGURE WINDOWS WIREWALL (if you have dumb stuff like Norton Internet protection installed you're on your own)
4...START -> Run -> type "firewall.cpl" -> OK
5...Klick on the "Exceptions" tab and then the "Add port..." button
6...Type "Teewars" (or whatever) in the name field and "8303" in the port field. Mark "UDP" and klick OK
The error message from the master server might still exist (and your server will then not be seen) but if you ask anyone to connect to your public IP address and they can connect you've done everything correct. See this thread for more info about the server settings and possible errors.
Good luck!
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