How to Unlock All Factions
1. Go to your "Medieval II Total War" folder.
2. Right click on the preferences.cfg file, select the properties tab and uncheck the read only box, then open it with notepad. This is so that you can edit the file.
3. Scroll to the bottom of the file.
4. Add the following lines:
[misc] unlock_campaign = true
5. Save and exit the file. You should now be able to play with all the factions next time you open the game.
Thanks to Aristocrat for creating the original article