The MP 620 does real well as a wireless printer but wireless scanning is another matter. I had it working somewhat with Snow Leopard and a different Mac (2008 MacBook). Canon has since updated drivers twice which has not impacted on wireless printing, however, wireless scanning is not working right. I kind of gave up on it because I rarely use that feature anyway.
The only thing I can suggest is try to run the Canon Network tools. The latest can be downloaded at the Canon site for the MP 620. Here is the
LINK.
I don't remember off hand which one of the utilities worked for me but here's the lineup:
Network Scanner Selector.app
Network Scan Utility.app
Network Tool.app
I believe I ran the Network Tool.app or the Network Scan Utility.app and that setup the MP620 to print wirelessly and use my WPA2 password. Then I think I ran the Network Scanner Selector.app. But like I said, that was for Snow Leopard. The same sequence does not seem to work for Lion, or at least reliably.