how do I use image capture to transfer videos from iPhone 5 to iMac

Connect the phone to your Mac with the cable and launch Image Capture. You should see the phone on the left under Devices. Click on the phone and that should allow you to import the videos.

You should use Photos to transfer videos from an iPhone. It is much more intuitive.

But if you want to do it with Image Capture, you have to first set your iPhone to open with Image Capture since it is set to Photos/iPhoto by default.

Your device should show up in Image Capture, but it may not show any photos or videos.

To change this, click the tiny arrow pointing up in the bottom left of the Image Capture window, then change "Connecting this iPhone opens..." and set it to Image Capture. Then disconnect the phone and reconnect it again.

This is the method for El Capitan. I don't know which OS you are using. If you want more accurate support please include all software versions being used with each device in question.

Best of luck