This happened to me like eight years or go, but it is still valid. After I successfully got my first work permit notification from my home country, the only thing I got from the Government of Canada was a notification letter with some directions.
Just after the airplane landed and I was able to recover my baggage, I did the line to pass with the agent. He asked me a few questions, why I was here? I answered to work, who I will work for? I said the name of my employer and showed him the letter. It is not really important if they know the employer or not. They are trained to discover lies, so it is more a matter of answering with the truth or triggering a red flag. After that, I was told to go to ail and wait to be called by a migration agent.
Not too much drama, I identified myself with my passport and gave him the confirmation letter. It has a UCI number, a number that is assigned to you and it will be used to keep track of all your immigration matters. Minutes later, I had on my hand my first work permit.