You may need to buy an SD card reader since not all Macs come with an SD card slot. If your Mac does have an SD card slot, it will be on the side of the casing (laptop) or on the back of the CPU box (desktop). It may also be on the side of the keyboard on some desktop units. Some devices require that you enable disk use via USB through its settings before your computer will recognize it. This is the blue face icon in your dock. It's in the toolbar at the top of the screen. This will open the Utilities folder, from which you can run the Disk Utility. Alternatively, you can hold down Shift and Command and then tap U to open Utilities. This app resembles a grey hard drive with a stethoscope on top of it. It should be listed under the "External" section in the left-hand pane of the Disk Utility window. If you don't see your memory card listed here, try removing and re-inserting the memory card. This is the stethoscope icon in the row of options at the top of the Disk Utility window. If you see a pop-up window that says "Your disk is about to fail" instead, you won't be able to repair your memory card. Once your Mac tells you the process is complete, you may safely remove your memory card and place it back in your camera. You may see an error entitled "The underlying task reported failure" here. If so, try restarting your Mac and running the repair again.

Summary:
Connect your memory card to your Mac. Open your Mac's Finder. Click Go. Click Utilities. Double-click Disk Utility. Select your memory card. Click First Aid. Click Run. Wait for your memory card to be repaired.