Unfortunately, it doesn't always take you to the Lobby channel. So the first thing to do is press <Change/Create Channel> and then go to the Lobby channel.
Sadly, this game doesn't have the player counts that it deserves, but there are still many games to be had. Your best bets are to come on during European and North American prime time, say from between 2 pm and 11 pm U.S. Eastern Standard Time. Also, most of the games, at least the 4v4 and 5v5 games, are for Entrenchment. (It's proven to be better than regular Sins.) Note that when you go to the Join Games window, you can only see games for the version of the game you are on. You cannot see Entrenchment games from Regular Sins and vice-versa.
