open your Rhye's and Fall of Civilization.ini file (located in your RFC folder), and change NoCustomScenario = 1 to NoCustomScenario = 0. THen you can open the "custom game" menu in the game.
I advise you NOT to play with other options (things such as always peace), or you could have nasty surprises: RFC is not done to tweak its options. But you can if you want to take risks.