To unlock all Factions in the game go to "C:\Program Files\Activision\Rome - Total War\Data\world\maps\campaign\imperial_campaign". Then open the text file "descr_strat". Then move whoever you want from unlockable or unplayable to playable. If you move some things to playable from unplayable they might crash the game if you try to use them or might just outright not work, so be careful.