Immigration does not cater to PR's who live outside Canada. Therefore, they will not mail it if they have already asked you to pick it up. Your options right now:
1. If you are visa exempt, go to Canada with your expired card and your letter to pick up the new one. They will let you enter.
2. If not visa exempt, but able to travel to the US, go to the US and cross to Canada by land.
3. If not visa exempt and unable to travel to the US, apply for a PR travel document at a Canadian embassy so you can pick up your card.
If you do not go to Canada and pick up your card within 6 months, they will send it back to headquarters and have it destroyed and you will have to apply again.