Hey,
I red&tried all the methods to find out the problem of Garmin Express when i tryed to add a new device.
FINALY! The problem is from the new version (7 i guess) of the Garmin Express and not with your system or You did something wrong. No! The Garmin team needs to make some fixes to resolve this problem.., buy the way.
what i did, i deleted the new Garmin Express program and reinstalled an older version and WORKS!
a link from you can download the driver for an older version:
https://www.dropbox.com/s/dw3vfeu2y9mu77e/GarminExpress.exe
hope it works! For me was a perfect solution