I am not sure if this is this right place to ask this question or not, but here goes. After installing Mountain Lion I now get a popup every time I login that says "Would you like to download and install software for “Canon MX860 series”? (I do have an MX860 Canon printer). Choices are Not Now or Install. If I click Install I get taken to a License Agreement page, after clicking Update, a Software Update windows appears with a progress bar that it is "Finding Software". Then about a minute later another message appears "The software is currently unavailable. You can check for the software later by choosing Software Update from the Apple menu. The Apple menu Software Update item shows a blank page with "No Updates Available" displayed.
Thanks for any suggestions,
If your Mac is connected to the Internet, disable the network connection and try to add the printer again.
If this does not help, disconnect the printer USB cable and then open Finder and navigate to HD > Library > Printers > Canon and trash the BJPrinter folder. Then empty the Trash and turn the network connection on. Last action is to connect the MX860 USB cable to the Mac again. See if this succeeds in getting the printer added.