I annoyingly had a OneNote notebook I used at work on a PC, but I couldn’t get it to sync into OneNote on my Mac.
Here’s the secret – it won’t automagically appear, you have to manually open the notebook.
Click on the plus sign in the Notebook dropdown.
In the popup window that opens, click “Open”, then select OneDrive or wherever your Notebook is synced, and then choose the Notebook you want to open.
That’s it! Kind of weird why Microsoft would not just make all Notebooks sync automagically, but well, its Microsoft.