Go to the map where you installed the game, then go to data/world/maps/campaign/imperial_campaign. There is a file called descr_strat, open it and you see the playable and un-playable factions, just move the unplayable factions to the line playable and see, you can start with the Gauls or Cartahge or the Senate and so on. Don't know if you play the Senate you can command the other Roman factions. Good luck!