Snow Leopard
After upgrading from Mac OS X v10.5.x to Mac OS X v10.6 (Snow Leopard), due to the OS specifications, the printer driver is deleted.
Mac OS X 10.6 printer drivers are available for the following models.
MP240
MP480
MP620
MP800R
MP970
MP980
MX310
MX320
MX330
MX700
MX7600
MX850
MX860
Download the printer driver supporting Mac OS X v10.6 from this site, and install it.