If you're using a courier (FedEx, DHL, etc), then use the 49 Dorchester St. address. If you're mailing the application by ordinary mail, you can use the PO Box.
I would recommend using the courier.
Family class
- Mailing address for applications to sponsor spouses, common-law partners or conjugal partners and dependent children (currently living outside Canada), other relatives and adopted children:
CPC Sydney
PO Box 9500
Sydney NS
B1P 0H5
- Courier address for applications to sponsor spouses, common-law partners or conjugal partners and dependent children (currently living outside Canada), adopted children and other relatives:
CPC-Sydney
49 Dorchester Street
Sydney, NS
B1P 5Z2