Hi Cyril,
Synchron Player is simply another AU plug-in, should appear in your sequencer right away.
Simple workaround to use it as a standalone: Vienna Ensemble / PRO.
If it doesn't show up in Logic, try to validate it manually in Logic's Plug-in Manager.
And if that doesn't work, you can celar your Audio Unit cache like this (it will make Logic rebuild it's plug-in Au cache):
1) In the Finder, choose Go > Go to Folder from the menu.
2) Type ~/Library/Caches in the "Go to the folder" field.
3) Press the Go button. Remove the "com.apple.audiounits.cache" file. Remove the ~/Library/Caches/AudioUnitCache folder.
EDIT: Are you on High Sierra, by any chance?
Best,
Paul
Paul Kopf Head of Product Marketing, Social Media and Support