After you have registered for your PaySync account, click on Settings.  You will now see a PaySync Authorisation Code.  Copy this code and paste it into the PaySync Settings in your Aussie Time Sheets software (Setup > General > Payroll Settings > select PaySync in drop-down):

Your Aussie Time Sheets software is now linked to PaySync.  You can now link and configure any available cloud payroll account within your PaySync account.