Larian Studios Forums Divinity: Original Sin 2 Divinity - Original Sin 2 - Help/Tips/Tricks Keeping the black cat alive

Unfortunately, with the Nine Lives mod you need to get the cat out of Fort Joy before getting the summon spell book and whistle for him.

For the App Store, you should be able to copy/extract the pak file for Steam/Nexus mods to the folder:
/Users/<UserName>/Library/Containers/com.larian.dos2/Data/Documents/Larian Studios/Divinity Original Sin 2 Definitive Edition/Mods