Most modern DSLRs have a feature called "PTP" which is used to remotely control the camera via USB, make images and download them onto the computer. This feature can be pretty handy if you are into stop motion animation or time-lapse photography. iStopMotion is a software from Boinx Software which does pretty much everything you need to make such movies. Sadly Canon DSLRs are not supported by Apple, so they also don't work in iStopMotion. But there is a pretty simple way around this limitation.
With every Canon DSLR comes the EOS Utility. This piece of software allows you to control your camera and also display the live image. This is pretty handy, but sadly doesn't do anything like onion-screening or something like that. This means it is pretty much useless for stop motion.
But what you can do is using the preview image in iStopMotion using a third party software called CamTwist. This way you can see what has changed in the image in iStopMotion and make the image itself in the EOS Utility. (And also in iStopMotion)
When you are finished you can then simply drag all images you have made into iStopMotion and your stop motion movie is ready for playback. The EOS Utility already saves all the images onto your computer, so you don't have to copy all the images from the camera.