If you don't have an address at the time of your landing, you can tell them that you will provide one later. You can notify them of the new address by changing it online, by calling the call centre or I have heard of people who landed who were given a fax number for that.
After you have arrived, you can set up a street address with UPS where they will forward you whatever mail you get to your home address that you give them. Some have said they have used a P.O. Box and it was accepted by CIC and you can also set up with Canada Post that they forward the mail from your P.O. Box to your home address that you give them. If you have friends or relatives in Canada, you can use their address but if the IO asks you on arrival if you will be staying at this address when the PR card arrives in the mail, should you say no, the IO may decide to withold the PR card until you have a permanent address of your own.
Worst case scenario, if you do not have an address to get the PR card sent to and must leave again to take care of things back home, you can apply for a travel document to get back into Canada and once you are back to stay, you can get the PR card sent to you. However, if that is more than 6 months after your landing, you would have to apply for the PR card, pay $50 and provide new pictures.