I got iPronto Todo working.
Merchon, the app developer, responded to my support email. I appreciate his help. Two steps:
1. The old Mac app/preference pane, iProntoSync, needs to be deleted. That entailed using Terminal to make the library folder visible and then deleting the pref pane: iProntoSync.prefPane in the root PreferencePane folder.(iProntoSync used WIFI to sync with iPronto ToDo before iCloud)
2. Delete iPronto ToDo off the iPhone and then reinstall via iTunes. Enter the iCloud ID and password in iPronto Todo iCloud/CalDAV settings, and tap "use iCloud." The app populates the Server URL, port, and path. Choose preferred setting for "Auto Sync Every".
The spinning circle in the top bar indicates it is syncing with iCloud.