I see there is a closed thread on this topic already but since I can't comment on it I'll try here. I am also having the problem where garmin express fails to install on my mac. I'm running El Capitan and it says "verifying install garmin express" but it never loads.
I am sorry that you are having trouble installing Express. This is not an issue that am familiar with so I don't have a quick solution for you. If you have an older version of Garmin Express, try uninstalling that one first. Also, make sure that you have restarted your Mac. If you are still having trouble, it would be best to contact Product Support by phone.
