If you added/created things through paperui, then you don’t have any item files. They’re stored in an internal json database.
Never used OH desginer (never got it working properly) so can’t help you with that part. May be that it doesn’t support working with OH2 paperui added items/things.
What version of designer are you using? If you are using v0.90 then you will need to revert to v0.80 as there are problems with the latest version and I think this was one of the issues.
Rules are created in the standard way with .rules files. I usually get the item names through the item section in habmin.
Or, you can install the experimental rules extension. Available under extensions->misc in paperui. I’ve just started messing with that for simple rules.