According to the specs, the new iMacs (mid 2010) support SDHC and SDXC; you might want to check to make sure yours supports them - find the specs for iMacs here (I'm being cautious here because your profile indicates you're running 10.5 = a bit older model):
http://support.apple.com/specs/#imac
If the built-in slot doesn't support the new cards, get a _high speed_ USB reader that will - that should solve the problem; but it might be advisable to contact Cannon support; they're real good about answering support emails.
As for importing pics or videos - here is a slightly different perspective: I prefer simply dragging (importing) them on to the desktop. The reason for this is:
1. videos: depending on the format your camera uses for videos, iMovie may or may not recognize it; I've had that with two Canons. In those cases, I've used MPEG Streamclip (free) to convert them and it's a pretty lossless process if you pick the right conversion format. For some formats, MPEG Streamclip requires the MPEG2 Component available from Apple ($20).
2. Pictures: I prefer making an initial assessment of the pics by looking at them with Preview and then decide which app I want to use (if any) out of several - or, they may wind up in the trash immediately!
So, you can use any of the built-in OS apps for import, but do not need to - you can also simply drag them from the icon onto your desktop and then decide what to do with them.