Having the MP780 installed previously will not be the cause of the ds file missing. It should install regardless so I believe it would be related to a fault with the driver installer. If you open Console and select the Install.log file, there may be entries for the scanner driver installation and an indication why it did not install the ds file.
Also, did you use the driver included on the CD that came with the machine or get if from the Canon web site. If the former, then download the driver from Canon and run that.
Well, I after reading about both of the proposed solutions and trying a few things, I took the sage advice of my past business partner, "If you don't like the answer you get from customer service, call again and talk to someone else."
So that's what I did with Canon. The first person was wrong. There was nothing wrong with the system nor a need to call Apple. Somewhere in on of the scanner set-up steps there was a need to specify that the scanner had a wireless connection.
Sorry to bother folks here and that I didn't make better use of your good ideas.