Have you previously imported images from this particular camera to this particular mac?
Is iPhoto up -to-date?
If not, it might not recognize your camera, as the profile for it is missing in iPhoto.
Also: have you tried open iPhoto, and instruct it manually to import?
(I need to say I never used iPhoto but I use experience from other photo software)
Check the iPhoto preferences. What does it say what iPhoto does when you connect a camera?