How to add new iPad to "registered devices"

I have OmniFocus on my MacBook Pro, iPhone, and iPad. I recently got a new iPad. When I add items to OmniFocus on the iPad, it doesn’t show up on the MBP or iPhone. I found the Setting “Registered Devices.” I only see my MBP and iPhone listed, and there is only a way to delete devices, not add them. How do I get my new iPad to be recognized as a Registered Device?

Is the new iPad logged into the same sync account as the other 2 devices?

Yes. It is logged into the same sync account.