Hi roman dacian,
Unfortunately, a nasty little bug introduced in the 1.3 patch, which is what BI does to your game, disabled the conventional method of enabling a faction to be playable. However, it is resolvable! May I suggest, to resolve it you can download the latest 1.6 patch. The horrible bug was fixed in the latest version so there will be no need to worry about it emerging it's ugly head!
Alternatively, should you not desire to download the latest patch, you can simply adding a description for each non-playable faction in your "C:\Program Files\Activision\Rome - Total War\data\text\campaign_descriptions.txt" file. You can do this by simply typing this in the file two lines below the lowest faction in the list:
{IMPERIAL_CAMPAIGN__[factions code name, see bottom of post for list]_TITLE}[enter faction's on screen name]
{IMPERIAL_CAMPAIGN_[factions code name, see bottom of this post for a list]_DESCR}[place your description here]
After you have inserted the text you can repeat with the next faction till you have a complete list of described factions! BTW, Pontus and Macedonia do not require descriptions adding as they are already there.
Hope this helps, cheers!
Non-playable faction code names that will need descriptions to be operated:
romans_senate - S.P.Q.R
armenia - Armenians
dacian - Dacians
numidia - Numidians
scythia - Scythians
spain - Spanish
thrace - Thrace
slave - Rebels