Is this a registered mail? or regular mail? Try pay extra attendtion at the common areas where people collect mails or discard mails. If it is a regular mail, it may be left at the common area, and waiting to be claimed. Also, informed the office or the superintendent ( I will try the office first) about you have an important mail coming in, and the sender forgot to write the apt number. And if they see it, ask them to inform you.
Pay attention to Canada post pick up notice (small paper slip) hanging around the common areas in your building.
Phone Canada post or ask people in the building about which postal outlet resonsible for parcels and registed mails picked up for your address. Chances are the registered mail will be at the postal outlet and waiting to send back to the sender. Once you find out the outlet, go there, and ask them to look it for you. Just keep trying on a daily basis, and it wil show up.
Just keep checking the building mail areas and the postal outlet on a daily basis. Talked the rental office about your situation, and ask them for the postal outlet which handle mails for your building.
Even though you are not officially move in yet, but I guess they would give you the key ahead of time, if they don't you can always have someone to let you in.