I have an interesting observation. I have a 5k iMac as well as a MacBook Pro.
The iMac is running macOS 10.13 and WD Sync seems to work although I had to uninstall and reinstall to get it working. I can select and add folders to the sync process.
The MacBook Pro is running macOS 10.13.1 beta 5 (probably the last beta before final release). I cannot get the WD Sync to work on the MacBook Pro. I have deleted and reinstalled with no luck. I have the same issue as others that the folder selection is grayed out.
The key difference between the 2 devices is that the MacBook Pro has the SSD formatted with the new APFS formatting. The iMac is standard formatting as APFS does not support a fusion drive (probably lucky for me). I do not believe running macOS 10.13.1 is the cause.
Curious if there is anybody with a MacBook Pro, macOS 10.13, and selected not to convert the SSD to APFS during the upgrade process. Are they having any issues?