Hi there,
The PIXMA drivers cannot be added by browsing to Canon in the Print Using menu. For USB connected PIXMA's, when you select the printer in the broswer list, the Print Using menu should automatically change to Canon IJ Printer.
What you can do is check that you have communication between the printer and the Mac. In the Finder, click on the HD icon and then open Library > Printers > Canon > BJPrinter > Utilities. Open the Canon IJ Printer Utility. The Printer List window that appears has a menu for selecting USB or Network connectted printers. With USB selected the iP6700D should be displayed with a Type of Canon IJ Printer. Select it and click on Maintenance. For the new menu, select Test Print and a Nozzle Check. If everything is okay you should get a page from the printer so you can Quit the utility. If you don't get a page then you may have a problem with the printer. Turn it off by unplugging the power cord but leave the USB cable connected. After a few seconds reconnect the power cord and do the test again. If you still get nothing then take the printer back to the store.
If you did get a page you have proven the comms are okay. Go back to the Printer Setup Utility, click Add and in the Default Browser view you should see the iP6700D. Select it and wait. Next to the Print Using menu you should see the 'wait' animation for a second or two and then Canon IJ Printer should appear. When it does, click Add to finish the install.
If the Print Using > Canon IJ Printer doesn't appear please reply and we'll work on the next steps.
Regards,
Paul