Yeah, you need to be a LOT more specific.
If you want to place a city on an existing game's map, or edit what buildings are in a given city, that's FireTuner.
If you want to create a scenario where a bunch of cities are pre-placed, that's WorldBuilder.
If you want to change how cities work in general, like their growth equations or how fast their defense ratings scale, that's ModBuddy's XML.
If you want to change what happens when a city is founded or captured or something, that's ModBuddy's Lua.
So explain exactly what you want to do, and people can then tell you whether it's possible and if so, which tool to use.