I have Canon Pixma MP560 wireless printer and scanner. Have you installed the Canon software and drivers on your iMac? You can go to Canon website and find the software based on your printer model. Maybe this is what you need: http://www.canon.co.uk/Support/Consumer_Products/Products_OLD/MG8140MG8150.aspx? type=download&page=1
You can filter by software and by your operating system. I typically install all of them except the My Image Garden because as far as I know it's for organizing photos.
After installing, the Wireless Setup Assistant can be used to help you connect to your printer. Or you can also add/remove printers and scanners in your System Preferences.
The printer and your iMac need to be connected to the same router so they are on the same LAN.