It's virtually impossible to open a Finder window and navigate to the right folders on a Mac.
Instead, open up the game and click on the settings button in the bottom left of the start screen. Choose the troubleshooting tab, and then click the button to Open folder containing Customizable Data. That'll open a Finder window the right place, and from there you can open the bb_cards folder, the templates folder, and then the ootp_mac folder.
That's where the xml files need to go. The images go in the images subfolder in that one.
More instructions
here.