My wife and I now have 3 cannon cameras only once did I install their software. Most of it is not very good. It is meant for some one that has absolutely no photo imaging software. Both iPhoto and and Aperture 2.0 even Apple's image capture, should have no problem with the raw output from your Rebel XSi. which I believe is .cr2 raw
For great photo editing for both raw and .jpeg, just use iphoto. Until you can get aperture or photoshop elements. Photoshop elements is pretty sweet for 79 to 99 bucks.