Alana,
Do you have a 600D or a 6000D. I ask because I have the 6000D and I am printing to it fine from Leopard using the Canon driver that was included with Leopard.
If you installed a driver from Canon, it could be conflicting with the driver that is preinstalled. So I would delete it from the printers list and then reset the printing system, which you do by Ctrl-click on the printers pane in Print & Fax and then selecting Reset printing system. Then unplug the printers USB cable, restart the Mac and open System Preferences > Print & Fax again. Then reconnect the USB cable and see if the printer appears. If it doesn't, click the + then Default. You should see the Canon listed as a USB device and when you select it the Print Using menu should change to Canon IJ Printer. If it does, click Add to complete the installation.