There should also be a button labeled: 'Scan-from-operation-panel-settings'. If you run this, your printer should show as one of the options to select.
If you install all the latest Yosemite versions of Canon drivers and utilities from canon support site, you should have something called 'Canon IJ Network Scanner Selector EX'. I have found a workaround to trigger scans from the printer, but it is MUCH less convenient than it was in Mavericks.
I have noticed the same issue you are talking about.
