To anyone else who runs into this same problem…
I got the exact same behavior - the add-on got installed but then it was not displayed in the available gadgets list on the Dashboard. In my case, the example included a remote condition, and remote conditions are no longer supported, so Jira silently ignored my dashboard item. I replaced the remote condition with “user_is_logged_in” and it worked. (see Does Jira Cloud no longer support callback-based conditions? - #2 by alan.parkinson)
Hope this helps