Aight, so I hit that link and followed the steps and it works perfectly. Just make sure your headphones or 1/8” aux cable is plugged in the headphone port when you create the aggregate. Otherwise, the system only sees the speakers.
When I opened Cubase, I just had to select the new aggregate from the cubase opening pop-up menu (while the headphones were plugged in) and voilà.
Thought I’d report back. 