Follow these steps to extract The Binding of Isaac: Afterbirth+'s resources:

A.	Open the folder containing ResourceExtractor in one Finder window and 
	leave it open.

B.	1.	In Steam, right-click on The Binding of Isaac: Rebirth, and select 
		Properties.
	2.	Click the Local Files tab
	3.	Click Browse Local Files
	4.	Right-click the app and select View Package Contents
	5.	Browse to Contents/Resources/
	6.	Leave this window open.

C.	Create a new folder "Extracted Resources" (you can name it whatever you 
	want) in the location where you want to extract the resources, and open 
	its location in a third Finder window. You'll need roughly 600 MB of free 
	space in this location.

D.	Now, open the app "Terminal," located in Applications/Utilities.

E.	1.	Drag "ResourceExtractor" onto the terminal window
	2.	Drag "Resources" (the parent folder, not the "resources" folder inside)
		onto the terminal window
	3.	Drag "Extracted Resources" onto the terminal window
	4.	Click on the terminal window and press return or enter to start the extraction