A link to the plugin would be great.
Did you start a new game after adding the plugin, or did you resume a previously saved game? If the latter, try starting a new game. A lot of scripts/plugins do a certain amount of setting up stuff when a new game is started (to save time doing it repeatedly while you're playing), and that setup stuff is then saved when you save your game. So if that hasn't been done, because the plugin wasn't there when you started the game, it will cause an error like that.