On MBAir first install created the Sync folder, but no apparent activity;
then I removed the app, reinstalled on Applications/Utilities, rebooted;
still does not show on the menubar (the others do);
still does not appear in Activity Monitor (the others do);
Having entered the respective Identification strings on the iMac and MBPro, and hit Rescan All, there is no apparent activity so far (about an hour);
Each machine’s Remote Devices says “Connected (Unused)”
Please post screenshots of the Syncthing Web GUI from all devices. Specifically, please make sure to include the part under “This Device” with “Listeners” and “Discovery”. You can also click both of the two and post screenshots of what you see there.
Please ignore the above. As explained below by @chaos, your devices are connected but now you need to share folders between them.