Download the NWN2 Client Extension and start NWN2Launcher.exe. You will be asked to install skywing's client extension. Click Ok.
Restart the game.
Create your personal login and select multiplayer internet. Search under "roleplay" category for Sigil - City of Doors (or direct connect to: scod.game-server.cc) and connect. You will have to auto-download about 1,7 GB before you can play. So make yourself a coffee and relax ;).