So Just Add This Line Of Code 
"+PlayableCharactersV2=(BaseId=0, Id=X, Name="Batman_Catwoman", bBad=false, Mask=0)"
To Your PCGame.ini file Which is Located At
"C:\Program Files (x86)\Steam\steamapps\common\Batman Arkham City GOTY\BmGame\Config\PC"
But You Will Probably Need The Community Patch For This Mod To Work 
"https://www.nexusmods.com/batmanarkhamcity/mods/1"
And Just Drag And Drop The File Into Your Main Arkham City Directory which is located In
"C:\Program Files (x86)\Steam\steamapps\common\Batman Arkham City GOTY"
Or Where You Have Your Games Installed On Your Driver