Why am I missing world leaders in Civilization IV: The Complete Edition?

I recently got Civ IV: The Complete Edition for my birthday, and a friend was looking at one of the expansion packs and talking to me about it, but when I looked to start a new game, I did not have all of the leaders from Beyond the Sword that he told me about. Were some of them not included, or were some restricted to certain map sizes and difficultly levels, or included as unlockable content?