Let me Explain, if you use the steam Verision, you just let steam install the game, Dont install anything else than the multiplayer client.
In this you start by istall game thrue steam, when its done you start the game in singleplayer, then after that install multiplayer client.
Non steam users, install game start it up in Singleplayer, Install patch , install client , Connect to multiplayer.
if None of thoose work as it should, you have errors, eather in game or patch, or in the windows. Send print screens on your errors in the topic, would be more easy to understand and help.