You must edit the game files before this faction is playable. To do so:

Open your main M2TW directory, then mods, teutonic, data, world, maps, campaign, imperial campaign, then the file called 'description_strat'. Edit the top section so it looks like this:
Code:
campaign		imperial_campaign
playable
	teutonic_order
	lithuania
	denmark
	novgorod
	mongols

end
unlockable
	hre
	poland

end
nonplayable
	norway
	kalmar_union
	slave
end
The Mongols do not have any victory conditions when modded into play.