Update:
Since the latest version of Arma3Sync it adds a "(synchronized)" to the modsets/events after you updated the according repository.
(and they changed it so it doesnt matter if you added the modset via the modsets button or via the join server list)

(this picture shows the modsets after I updated the normal repository, the joint tiger repository wasnt updated and could use a old modset)
So, the next time your modset isnt updated just go to the repository and update the repository (you have to hit the "play" button, even if there are no updates to the files).
As soon you see the "(synchronized)", youre good to go.