Create a new network configuration for your PS2 and name it "taa" (you can name it anything you want, this just makes sense since its going to be for Tribes).
Configure everything as you normally would to connect online for your network, but for the Primary DNS field, enter:
You can use 0.0.0.0 for the Secondary Field.
Boot up Tribes Aerial Assault, choose the "taa" network configuration, and connect.
If successful you should see a dummy server in the list.
Currently the best experience for TAA is playing online with bots. The AADS dedicated server ran into some Windows compatibility issues after some service packs, and some testing is needed to determine the optimal environment to run AADS. Also, when hosting from a PS2 you get bots which are absent from AADS. So hosting from a PS2 is a lot better due the trouble of finding lots of players to populate a TAA online session.