C:\Program Files\Activision\Rome - Total War\Data\world\maps\campaign\imperial_campaign. Open the "desc_strat" text document in that folder and cut-paste the faction names from "unlockable" and "unplayable" to "playable". BI's the same, just in the equivalent folder (Data\world\etc.) in the BI folder within the main RTW folder.
Some factions screw up the game, though, so be careful- don't try and play as the Rebels, or the BI shadow factions like the Roman Rebels or the Ostrogoths. I'm not sure how the emergent factions in BI- Slavs and Romano-British, IIRC- effect the game, and the Senate from RTW will work, but it completely removes the special senate functionality from the game and will crash if you go to the senate screen.