Sync seems to be stuck on `not yet synchronized, upload database`

[BLUEFROG]

1. The first place you should always look when having a potential sync issue is Window > Log.
2. Bonjour is not only covered in the Help referred to by @rmschne, but I did a more exhaustive dive here:

Bonjour Simplified

To help clarify the conditions under which Bonjour can be used, as well as show the simple steps to setting up such a sync, here are notes and steps to follow. Requirements Bonjour can be used to Sync between devices, IF: The devices are on the same network, noting it will not work over VPN. The network is private or one that allows Bonjour connections or non-standard ports. (Public and corporate WiFi sometimes disallow these.) Your firewall or an application like Little Snitch is Off or has …