FYI: I've called the SYNCING.NET support (very helpful).
Here is a step by step description.
To open and manage SYNCING.NET in the rule window, do the following:
# Open the main application window
# Click the Firewall button
# Click the Rules link
# In the left part of the window click the + button
# Add the processes SyncService.exe, SyncingOLWatchService.exe and NotificationService.exe in the folder path "C:\Programs Files\SYNCING.NET Technologies\Syncing.net\bin".