I connected a USB cable from MM to my Mac, but it doesn’t appear as a volume, so I can’t save patches directly to it.
That’s right, the MetaModule does not show up if you connect it to a computer via USB. Instead, save patches on a USB drive or SD card and then put them into the MetaModule.
Make sure you save patches using the red SAVE button the MetaModule inside VCV Rack: here are instructions and screen-shots showing the process: