I downloaded the latest Fortress of Doom from TheTain.org but after putting it in my plugins folder and then choosing New Game in multiplayer mode, and then activating it, no game shows up.
when you're in the plugin selection window and about to press 'ok' to load the plugin, hold down shift as you click - this will force solo levels to appear in the list, the same way holding shift as you click "new game" forces all the levels (except for the secret ones) to appear for you.