Never showed you how to play online, you mean? Hmm, well, from memory... When connected to the internet, simply start MOHAA and then go to the multiplayer section (door on the right hand side of the room). (Configure your player name and skin as you see fit first, otherwise you take the defaults and have the name "Unnamed Player" or something like that) Then simply click on the door in the middle of the screen for joining a game, select the non-LAN option (Internet?), and unless you know a particular server address you should choose Browse Servers (or whatever). MOHAA then lists all the hundreds/thousands of servers and you basically choose whichever game you want. It's far easier than I was expecting! You don't need to join any groups or download any software.
If all this sounds complicated, it takes me just seconds to go to the room, choose the internet option, and then start browsing to find a game I want to join (a ping below 100, a good number of people playing, the right type of game for my mood, etc.). The multiplayer dimension adds a lot extra to the game, especially if you have a broadband connection! One word of caution: the tactics and style of play for a multiplayer game are completely different to the single player missions; expect to be repeatedly embarrassed at the beginning, but don't worry, you'll improve with time. I'm only just getting to the slightly less red-faced stages now!