Hey, first message here so: do not shoot at piano player.
I read the Vivoactive 5 manual in here , I go and download the Garmin Express app here , I install it, I connect my Vivoactive 5 to my mac and:
Your device is not supported. WTH 
Someone else out there? (or what I am doing wrong?)
Note: I live in Europe (France ATM)
Bye.