Hi Susan,
Please follow the steps below to run "Start-OnlineCoexistenceSync".
1. On the computer that is running the Directory Sync tool, navigate to the directory synchronization installation folder. By default, it is located here: %programfiles%\Microsoft Online Directory Sync or %programfiles%\Windows Azure Active Directory Sync (the location depends on the version you have installed).
2. Double-click DirSyncConfigShell.psc1 to open a Windows PowerShell window with the cmdlets loaded.
3. In the Windows PowerShell window, type Start-OnlineCoexistenceSync, and then press ENTER.
Thanks,
Edward Qu