RLM Arena and Simulation MotorSports Lobby (SMS Lobby) are server hosting services that many leagues in NR2003 utilize. They both also offer a cheat detection layer making them popular with many leagues.
Drivers are typically required to connect through either service in order to access the league's game server.
RLM ARENA - Once you download the application below, Sign in with the same login information from the leagues site you chose. When you are ready to race, find your league in the list and start the game.
SMS Lobby - Once you download the application below, You should be prompted to select your NR2003 player name/directories and other basic settings. When you are ready to race, find your league in the list, enter the server password and click "launch".