How do I get pictures from a CF card into my iMac?

You have two options:

1: Get a USB->CF card reader.

2: Insert the CF Card back into your camera, and plug your camera to your Mac using a compatible USB cable. You might have to select a setting on your camera to initiate the connection. Many don't require it, but know that going in.

you can shop at OWC and get a 3-Port USB 3.0 Hub With CF/SD/TF Card Reader. they test the hardware they sell on macs so you know they work as they should.

By buying a CF card reader you can plug into a USB-A or USB-C port on the Mac. Every tech supplier has them.