There is no driver for the OS Version you selected. The driver may be included in your OS or you may not need a driver.
There are some SW there though.
Only Pixma drivers included in OSX10.5 are...
Model Version PIXMA Pro9000 10.1.1.0 PIXMA Pro9500 10.1.1.0
Might see if one of those work.
Other than that, might try the MX350 drivers, says 10.4/10.5, etc., but doesn't say if it's for PPC or Intel architecture...
Are you at 10.5.8 already?
At the Apple Icon at top left>About this Mac, then click on More Info, then click on Hardware> and report this upto but not including the Serial#...
Model Name: iMac
Model Identifier: iMac7,1
Processor Name: Intel Core 2 Duo
Processor Speed: 2.4 GHz
Number Of Processors: 1
Total Number Of Cores: 2
L2 Cache: 4 MB
Memory: 6 GB
Bus Speed: 800 MHz