In order for the magicJack device to work with Mac OS El Capitan/Sierra please follow the below steps:
Download the Softphone application
Download the Softphone application here
Double-click the Softphone application you just downloaded “magicJackSetup.dmg”
Download the new firmware update
Download the new firmware updater here
Plug the magicJack device into Mac computer (do not unplug until Step 5)
Double-click the firmware file you just downloaded “mjisoupdate.dmg”
Terminal will indicate the upgrade process has started,  the upgrade process should take less than one minute. In Terminal you will see “[Process completed]”
Unplug your magicJack device and then plug it back in