Muppet?

To unlock Parthia, go to Data/world/maps/campaign/imperial_campaign in Windows Explorer and open up the descr_strat.txt file with Notepad. Down a few lines is the Playable factions list, the Unlockable factions list, and the Unplayable factions list. Move all the unlockable factions to the playable list and all of the unplayable factions to the playable factions list (except the senate and slaves, unless you really like problems and crashes). Save, and play