This is how to add your Air Canada Aeroplan Number to United Airlines account.
Step by Step

Go to United.com https://www.united.com and sign in
Go to MY Account or View Account
Go to Profile (at bottom of page)
Edit Traveler Information (edit Traveler)
Go to bottom of page and find Frequent Flyer Information
Click on NEW and then add your Air Canada Aeroplan Number
Select program...Air Canada Aeroplan
Account Number: type in your Aeroplan Number here
CLICK SAVE...most important

now go back one page to Edit traveler Frequent Flyer information
Click on the Down Arrow where the United MileagePlus
Now you will see your Aeroplan Number added to the United Airlines Traveler information

thanks