I wanted to get the replay feature working too, but the replay toggle in the options window wasn't there. But I found the setting in UserInterface.opt that enables the feature (change the 0 to 1), and I can now turn it on in the options window.
/ ********************************************************
// Replay
// ********************************************************
inoCanUseReplay = 1 // replay is NOT allowed in current game engine, player can't activate it with inoReplay (stored in general.opt, shown to player in Options Window, Main Menu)
When using the LoadReplay command in the game I don't need to enter the full path, and it works just fine.