This is info for everyone who has such a printer. The following information is REALLY hard to find.

To get Canon's OWN printer status monitor working, you need to do the following, as superuser:

1. stop the cups printer daemon. (rccups stop)
2. edit /etc/cups/printers.conf.
Find your printer in the file, and change the line that says
DeviceURI usb://dev/usb/<whatever>
DeviceURI cnij_usb:/dev/usb/<whatever>.
Note the SINGLE slash after the colon.

This CANNOT be done via lpadmin or the web interface, since these give errors when using URIs with underscores.

3.restart cups. (rccups restart).

The should be done with your default user permissions.
start the status monitor for your printer.
If your printer is set as default, just run 'cngpijmon<model number of printer>'. for example, my printer is an iP3500. so my command is cngpijmonip3500. Some printers may differ in that you only have 'cngpijmon'. If it's not your default printer, follow it with a single space and your printer's name in CUPS:
cngpijmonip3500 myPrinterName

With the status monitor open, print something. You should see ink levels displaying, and your printer's status.

Just found this out, and I was hoping to share it with the community.