uh...help

[Alexander the Pretty Good]

That's a easy. Just go to your directory folder (Let's call it Rome - Total War).

Then go to the "Data" folder, then the "world" folder then the "maps" folder then the "campaign" folder then the "imperial_campaign" folder. In there you will find a file. It's called "descr_strat.txt" - open it up.

So the file path will be something like this:

Rome - Total War/Data/world/maps/campaign/imperial_campaign/descr_strat.txt

There you will find a list like this:

campaign		imperial_campaign
playable
	romans_julii
	romans_brutii
	romans_scipii
end
unlockable
	egypt
	seleucid
	carthage
	parthia
	gauls
	germans
	britons
	greek_cities
end
nonplayable
	romans_senate
	macedon
	pontus
	armenia
	dacia
	scythia
	spain
	thrace
	numidia
	slave
end

Just swap around the faction names. To unlock all:

campaign		imperial_campaign
playable
	romans_julii
	romans_brutii
	romans_scipii
	egypt
	seleucid
	carthage
	parthia
	gauls
	germans
	britons
	greek_cities
	romans_senate
	macedon
	pontus
	armenia
	dacia
	scythia
	spain
	thrace
	numidia
	slave
end
unlockable
end
nonplayable
end

However, the rebels ("slaves") and the Senate can be buggy to play. I'd recommend:

campaign		imperial_campaign
playable
	romans_julii
	romans_brutii
	romans_scipii
	egypt
	seleucid
	carthage
	parthia
	gauls
	germans
	britons
	greek_cities
	macedon
	pontus
	armenia
	dacia
	scythia
	spain
	thrace
	numidia
end
unlockable
end
nonplayable
	slave
	romans_senate
end

I'm 98% sure that's how it's done.

EDIT: Save a back-up of the original "descr_strat.txt" - just in case!

And have fun!