Apologies, i just fired up 1.3.4 here as well and the sync manager feature is not there.
It was in the 1.4 beta and which is no longer available to download on the forum.
The most important thing to know is that if you do add extra cues and stuff on the desktop app dont use this highlighted button. Best to drag from your crates/playlists panel to the USB.
Guess they have closed the public Beta, because there is a new one on the way, so they dont want people to start a new test-round and repport things that might already be fixed in the next Public Beta…
But its just a guess… I would wait for the next public one.