Have you updated it to version 1.11 of the game? You need to download and install Impulse and then register the game with Impulse. Then you can update it. If you don't have the most recent version then you can't log on to Ironclad Online. (And it's the Ironclad Online button you want, not the "Multiplayer" (for LAN games) button.) Look forward to seeing a new face online.
