I don't use any, but guessing that you need to access them through the Project backstage in the AddIn section
Dan, I forgo to mention that I have already added a folder through the Add-In Manager so that's not it.
When I restarted Pro I get a message in the Add-In Manager that may explain things.
"This add-in cannot be loaded because it targets an incompatible version of ArcGIS Pro. Current version: 3.0 Add-in targets: 2.5"
Nope, I now have an Add-In recompiled for 3.0 and have added the folder. Stopped and restarted Pro and still no Add-In tab.
I think it is created just when you have an AddIn that uses it.
You do not see it if its empty.
Tech Support would be best at this stage