Ok dude your going to need to delete your existing factions plugin from your plugins folder.
Next go download the 1.5.2 game version of mcore and factions The links are here: Mcore - http://dev.bukkit.org/bukkit-plugins/mcore/files/
and factions - http://dev.bukkit.org/bukkit-plugins/factions/files/
Make sure its the latest 1.5.2 version of these plugins. After that just start your server and it should be working!