To load the Azure AD Connect PowerShell module, use this command:
Import-Module "C:\Program Files\Microsoft Azure AD Sync\Bin\ADSync\ADSync.psd1"
For more info about this issue, see the Technet blog of Rhoderick Milne:
https://blogs.technet.microsoft.com/rmilne/2014/10/01/how-to-run-manual-dirsync-azure-active-directory-sync-updates/