In order for your Tribes server to be seen from outside of your network, you will need to forward the port that the Tribes server users.

Step 1
Open a command prompt by going to Start -> Run and typing cmd. On the commandline, type ipconfig and press Enter. There are two entries of importance, IP Address and Default Gateway. They should look similar to the following:

IP Address. . . . . . . . . . . . : 192.168.1.2
Default Gateway . . . . . . . . . : 192.168.1.1

Step 2
Open your web browser and type the Default Gateway address into the address bar and hit Enter. (ie. http://192.168.1.1) You will be asked to login. If you do not know the login credentials, consult your router's instruction manual. Once logged in, find the Port Forwarding section. For a Linksys router, it is under the Applications & Gaming tab. In the Application field, type a name for the service, anything will do. In the Start and End boxes, enter the port range that you want to open. For a normal Tribes server, you can enter 28000 in Start and 28001 in End. For the Protocol, choose Both. In the IP Address box, fill in the remaining boxes so that it matches the IP Address from earlier. (ie. 192.168.1.2). Place a check in the Enable box to enable the rule, and click Save Settings.

For other routers, visit Portforward.com for router-specific instructions.