I have an old Garmin 800 and trying to get it to run on the Mac is not simple. Here are the quick tips.
First you need the Garmin Express drivers, the easiest way for geeks is to run brew install Garmin-connect or go to the site and download it.
Then you want to connect the USB and then turn on the device, this sequence seems to be pretty important. If it works, the Garmin Express application should start and you will see two Devices boot up. One is called Garmin and then the other is the name of the SD flash card you have loaded.
You should see Garmin Express check for updates and then you can try sync. This will copy up the .fit files from the SD into Garmin Connect which is their online service. In my case, it kept saying connect failed.
Not sure why, but the fix is pretty easy. Go to the SD drive and look for the directory Garmin/Activities you should see a bunch of fit files, then start up Garmin Collect and drag and drop them into the website.