Hi,
First you need to go into:
C:/Program Files/The Creative Assembly/Rome: Total War/data/world/maps/campaign/imperial_campaign/descr_strat.txt
If you have Gold or Total War Eras.

If you have standard R:TW, not Gold or eras go into:
C:/Program Files/Activision/Rome: Total War/data/world/maps/campaign/imperial_campaign/descr_strat.txt

Then cut and paste the non-playable factions and the unlockable factions into the playable section. Remember to keep the tabs infront of the faction's code names! Good luck, enjoy playing as all factions and cheers!