Choose desired game mode (the "Mercury" "Jupiter" "Saturn" (etc.) thing is just the "room". Pick whichever, it doesn't matter) and confirm.
You should get a new window with a waiting status (should say 0/10).
Leave that window there, now open WindomXP.exe and select the last option in the title menu (i.e. netplay).
Where it says put IP, put in YOUR IP address (http://www.whatismyipaddress.com
if you don't know your own IP).
Press DOWN ARROW (mouse does not work here)
Where it says username, put anything (this will be your screenname online).
Choose pilot, mecha, etc.
Down arrow, when "Login" is highlighted, press enter.
Choose a room (There should only be one available, it'll say 0/10, choose that one. The others will be closed unless you opened multiple server.exes)
Give your IP to whoever you want to play (i.e. post it here) and get them to do everything in the netplay thing (IP, username, etc).
When they join, you can play. Server will still run if the game itself is closed (so keep Server.exe open so some others can play =x).