Well, my english is terrible, but i hope you could undestand me. Letīs go with it:
First, you have to be sure that you have the latest version (2.02) installed. Next, you enter the game, go to the Simulator and select Multiplayer. You will be asked to choose between two posibilities: Host Game (This mean that you are the server, you are the boss, and the other players joins YOUR game) and Join Game (You select this and you can join a game created by another host).
In both you have to select "Internet ON" and TCP/IP. If itīs your first time in multiplayer, i think itīs better you join another game. It will be easier to you.
The Internet Gaming Zone (www.zone.com) itīs a site which offers the posibility to play to many games...XWA included.
I hope I have helped you.