Try this..
I know the easiest way to do it on a pc is to update the drivers associated with it, but it has been years since I played with a Mac. I have xp loaded on a P4 so it is pretty easy..when uninstalled the right drivers automatically are re-installed by windows..and it it installs the external removable drive (g: in my case) as a removable disk drive automatically when I plug in the camera and turn it on and to "pc", and I load what pictures I wish to keep. I didn't load any software for the camera.
Do you have a drivers management console of some sort on your mac?
Check to see if you have a driver conflict or IRQ conflict with your USB port.
Go to wherever your drivers are managed and check to see if your USB is correctly set up and active. Uninstall and reinstall the drivers if necessary.
If your camera is loaded up as a hardware imaging source on your drivers tree update the drivers. It is best to get them from the manufacturer for your device and to see if there are additional tweaks you need to do for it to show up.
Check your USB cable.
Check to see if it is connected properly to your motherboard, inside.
Check to see if your digital camera is set to "computer" and on.
Check your camera batteries.