I have nearly the same setup as you. I print successfully from LR - with very good monitor-print matching - using profiles. When you choose the paper profile in LR's print dialog, the OS automatically selects Colorsync to manage the process, part of which is to turn off further colour management. All options are greyed out.
I can think of only three explanations for you dark prints when using profiles:
(1) they're not really too dark, rather the screen luminance is too high (see digital dog's contributions elsewhere on this forum on that issue)
(2) the profile you're using is not the correct profile for the paper you're printing on - did you create the profile yourself, or is it a 'canned' profile? supplied by?
(3) in the print driver you have somehow not selected the appropriate media setting (under 'Quality and Media' in the driver)
Do you get this dark phenomenon when you print on a Canon paper using their canned profile and the appropriate media setting? While a canned profile may not give you superb colour matching you shouldn't be getting dreadful lightness/darkness mismatches. If so, I would suspect explanation (1) above .
One of your screen shots suggests you may be using a Red River paper. If that's the case double check that you are using an appropriate media setting (ie which of the Canon media choices should you make to best approximate the reflectance, absorbency characteristics of your RR paper). If you get that wrong all hell can, and often does, break loose. RR should have suggested an appropriate media setting when supplying the profile (if that's where you got it).
BTW if you have not updated OS 10.6 to the last version, OS 10.6.8, I would suggest doing that. There were some issues with the initial releases of 'Snow Leopard'.
And, as suggested by Doyle, the latest Canon driver is known to work well with 10.6.8 (and 10.7 too!)
I hope this may be helpful