Here are the steps to download a FIT file from the Karoo using a Mac:
Download and install the Android File Transfer app on your Mac. Click here.
Enable Developer Options on your Karoo. Click here.
Under Developer Options, toggle "USB Debugging" on.
Connect your Karoo to your Mac using the USB Cable.
Open the Android File Transfer app on your Mac.
A popup message would appear "Can't access device storage", click "Ok".
On the Karoo, under developer options, tap on 'Select USB configuration'.
Select the MTP from the list.
(Tap on MTP from this list even if it is already selected)
Android File Transfer app will now show the folders inside the Karoo.
Open the folder named "FitFiles".
This folder will contain FIT files of all your Activities that are saved on your Karoo.
You can copy the desired activity to your Mac.