To allow customer, guest, and subscriber synchronisation to take place between Magento and Dotdigital, you must enable all of the Customer Sync, Guest Sync and Subscriber Syncs.
Enable customer, guest and subscriber synchronisation
Go to System > Configuration > Dotdigital > Sync Settings.
Under Sync, expand the Enabled drop-down menu for Customer Sync, Guest Sync, and Subscriber Sync and select Yes.
Select Save Config.
Magento confirms that the settings have been saved, and customer, guest, and subscriber synchronisation now takes place as scheduled between Magento and Dotdigital.
You can change the contact synchronisation batch size in the Developer section. The ideal number for batch size is dependent upon a number of factors and may need some trial-and-error to arrive at.
I've enabled the syncs but contacts aren't appearing in Dotdigital
It can take 15/20 minutes from enablement for contacts to appear in your Dotdigital account. After enablement, data needs to be batched and added to the importer and then the importer needs to process the import.
Learn more in Data syncing - Checking progress.